Advanced Cross-Chain Staking Portal Solidity, JavaScript, Web3

```solidity
pragma solidity ^0.8.0;

// Cross-Chain Staking Portal - Example Contract

import "@openzeppelin/contracts/token/ERC20/IERC20.sol";
import "@openzeppelin/contracts/access/Ownable.sol";

contract CrossChainStakingPortal is Ownable {

    IERC20 public stakingToken; // The ERC20 token being staked.
    uint256 public stakeRequirement; // Minimum stake amount.
    uint256 public totalStaked; // Total amount staked in this contract.
    uint256 public totalRewardsDistributed; // Total amount of rewards distributed.

    mapping(address => uint256) public stakes; // Mapping of staker address to stake amount.
    mapping(address => uint256) public rewardsOwed; // Track rewards owed to each address.

    uint256 public rewardRatePerBlock; // Rewards distributed per block per staked token.
    uint256 public lastRewardBlock; // Last block rewards were calculated.

    event Stake(address indexed staker, uint256 amount);
    event Unstake(address indexed staker, uint256 amount);
    event ClaimRewards(address indexed staker, uint256 amount);
    event RewardsDistributed(uint256 amount);

    constructor(address _stakingTokenAddress, uint256 _stakeRequirement, uint256 _rewardRatePerBlock) Ownable() {
        stakingToken = IERC20(_stakingTokenAddress);
        stakeRequirement = _stakeRequirement;
        rewardRatePerBlock = _rewardRatePerBlock;
        lastRewardBlock = block.number;
    }

    // ********** Modifier **********
    modifier onlyIfStaked() {
        require(stakes[msg.sender] > 0, "You have no active stake.");
        _;
    }

    // ********** Admin Functions **********

    function setStakeRequirement(uint256 _newRequirement) external onlyOwner {
        stakeRequirement = _newRequirement;
    }

    function setRewardRate(uint256 _newRate) external onlyOwner {
        rewardRatePerBlock = _newRate;
        lastRewardBlock = block.number;
    }

    // Allow the owner to sweep tokens accidentally sent to the contract.
    function recoverERC20(address _tokenAddress, address _recipient, uint256 _amount) external onlyOwner {
        require(_tokenAddress != address(stakingToken), "Cannot recover staking token."); // Prevent accidentally removing the staking token.
        IERC20(_tokenAddress).transfer(_recipient, _amount);
    }


    // ********** Staking Functions **********

    function stake(uint256 _amount) external {
        require(_amount >= stakeRequirement, "Stake amount is below the requirement.");
        stakingToken.transferFrom(msg.sender, address(this), _amount);

        updateRewards(msg.sender);

        stakes[msg.sender] += _amount;
        totalStaked += _amount;

        emit Stake(msg.sender, _amount);
    }

    function unstake(uint256 _amount) external onlyIfStaked {
        require(_amount <= stakes[msg.sender], "Unstake amount exceeds stake.");

        updateRewards(msg.sender);

        stakes[msg.sender] -= _amount;
        totalStaked -= _amount;

        stakingToken.transfer(msg.sender, _amount);

        emit Unstake(msg.sender, _amount);
    }

    // Claims rewards. This updates the rewardsOwed for the staker to 0 after sending
    function claimRewards() external onlyIfStaked {
        updateRewards(msg.sender);

        uint256 rewardAmount = rewardsOwed[msg.sender];
        rewardsOwed[msg.sender] = 0;

        require(rewardAmount > 0, "No rewards to claim.");
        stakingToken.transfer(msg.sender, rewardAmount);
        totalRewardsDistributed += rewardAmount;

        emit ClaimRewards(msg.sender, rewardAmount);
    }


    // ********** Reward Calculation Logic **********
    //  This core logic is the most important and complex part.

    function updateRewards(address _staker) internal {
        if(block.number > lastRewardBlock) {
            uint256 pendingRewards = calculateRewards();
            totalRewardsDistributed += pendingRewards;
        }

        // Calculate rewards for the _staker
        rewardsOwed[_staker] += earned(_staker);
    }

    // Calculate the total rewards earned since the last reward distribution.
    function calculateRewards() internal returns (uint256) {
        if(totalStaked == 0) {
            lastRewardBlock = block.number; // Prevents div by zero
            return 0;
        }

        uint256 blocksSinceLastUpdate = block.number - lastRewardBlock;
        uint256 rewards = (blocksSinceLastUpdate * rewardRatePerBlock * totalStaked) / 1e18; // Scale reward rate to 18 decimals

        lastRewardBlock = block.number;
        emit RewardsDistributed(rewards);
        return rewards;
    }

    // Calculates how much reward a user earned
    function earned(address _staker) public view returns (uint256) {
        uint256 blocksSinceLastUpdate = block.number - lastRewardBlock;
        uint256 rewards = (blocksSinceLastUpdate * rewardRatePerBlock * stakes[_staker]) / 1e18;
        return rewards;
    }

    // ********** View Functions **********
    function getStake(address _staker) public view returns (uint256) {
        return stakes[_staker];
    }

    function getRewardsOwed(address _staker) public view returns (uint256) {
        return rewardsOwed[_staker];
    }
}
```

```javascript
// JavaScript (Web3.js) for interacting with the CrossChainStakingPortal contract

// Assuming you have Web3 injected (e.g., via MetaMask)

// Replace with your contract address and ABI
const contractAddress = "YOUR_CONTRACT_ADDRESS";
const contractABI = [
	{
		"inputs": [
			{
				"internalType": "address",
				"name": "_stakingTokenAddress",
				"type": "address"
			},
			{
				"internalType": "uint256",
				"name": "_stakeRequirement",
				"type": "uint256"
			},
			{
				"internalType": "uint256",
				"name": "_rewardRatePerBlock",
				"type": "uint256"
			}
		],
		"stateMutability": "nonpayable",
		"type": "constructor"
	},
	{
		"anonymous": false,
		"inputs": [
			{
				"indexed": true,
				"internalType": "address",
				"name": "staker",
				"type": "address"
			},
			{
				"indexed": false,
				"internalType": "uint256",
				"name": "amount",
				"type": "uint256"
			}
		],
		"name": "ClaimRewards",
		"type": "event"
	},
	{
		"anonymous": false,
		"inputs": [
			{
				"indexed": true,
				"internalType": "address",
				"name": "previousOwner",
				"type": "address"
			},
			{
				"indexed": true,
				"internalType": "address",
				"name": "newOwner",
				"type": "address"
			}
		],
		"name": "OwnershipTransferred",
		"type": "event"
	},
	{
		"anonymous": false,
		"inputs": [
			{
				"indexed": false,
				"internalType": "uint256",
				"name": "amount",
				"type": "uint256"
			}
		],
		"name": "RewardsDistributed",
		"type": "event"
	},
	{
		"anonymous": false,
		"inputs": [
			{
				"indexed": true,
				"internalType": "address",
				"name": "staker",
				"type": "address"
			},
			{
				"indexed": false,
				"internalType": "uint256",
				"name": "amount",
				"type": "uint256"
			}
		],
		"name": "Stake",
		"type": "event"
	},
	{
		"anonymous": false,
		"inputs": [
			{
				"indexed": true,
				"internalType": "address",
				"name": "staker",
				"type": "address"
			},
			{
				"indexed": false,
				"internalType": "uint256",
				"name": "amount",
				"type": "uint256"
			}
		],
		"name": "Unstake",
		"type": "event"
	},
	{
		"inputs": [],
		"name": "claimRewards",
		"outputs": [],
		"stateMutability": "nonpayable",
		"type": "function"
	},
	{
		"inputs": [
			{
				"internalType": "address",
				"name": "_staker",
				"type": "address"
			}
		],
		"name": "earned",
		"outputs": [
			{
				"internalType": "uint256",
				"name": "",
				"type": "uint256"
			}
		],
		"stateMutability": "view",
		"type": "function"
	},
	{
		"inputs": [
			{
				"internalType": "address",
				"name": "_staker",
				"type": "address"
			}
		],
		"name": "getRewardsOwed",
		"outputs": [
			{
				"internalType": "uint256",
				"name": "",
				"type": "uint256"
			}
		],
		"stateMutability": "view",
		"type": "function"
	},
	{
		"inputs": [
			{
				"internalType": "address",
				"name": "_staker",
				"type": "address"
			}
		],
		"name": "getStake",
		"outputs": [
			{
				"internalType": "uint256",
				"name": "",
				"type": "uint256"
			}
		],
		"stateMutability": "view",
		"type": "function"
	},
	{
		"inputs": [],
		"name": "lastRewardBlock",
		"outputs": [
			{
				"internalType": "uint256",
				"name": "",
				"type": "uint256"
			}
		],
		"stateMutability": "view",
		"type": "function"
	},
	{
		"inputs": [],
		"name": "owner",
		"outputs": [
			{
				"internalType": "address",
				"name": "",
				"type": "address"
			}
		],
		"stateMutability": "view",
		"type": "function"
	},
	{
		"inputs": [
			{
				"internalType": "address",
				"name": "_tokenAddress",
				"type": "address"
			},
			{
				"internalType": "address",
				"name": "_recipient",
				"type": "address"
			},
			{
				"internalType": "uint256",
				"name": "_amount",
				"type": "uint256"
			}
		],
		"name": "recoverERC20",
		"outputs": [],
		"stateMutability": "nonpayable",
		"type": "function"
	},
	{
		"inputs": [],
		"name": "renounceOwnership",
		"outputs": [],
		"stateMutability": "nonpayable",
		"type": "function"
	},
	{
		"inputs": [],
		"name": "rewardRatePerBlock",
		"outputs": [
			{
				"internalType": "uint256",
				"name": "",
				"type": "uint256"
			}
		],
		"stateMutability": "view",
		"type": "function"
	},
	{
		"inputs": [
			{
				"internalType": "address",
				"name": "",
				"type": "address"
			}
		],
		"name": "rewardsOwed",
		"outputs": [
			{
				"internalType": "uint256",
				"name": "",
				"type": "uint256"
			}
		],
		"stateMutability": "view",
		"type": "function"
	},
	{
		"inputs": [
			{
				"internalType": "uint256",
				"name": "_newRate",
				"type": "uint256"
			}
		],
		"name": "setRewardRate",
		"outputs": [],
		"stateMutability": "nonpayable",
		"type": "function"
	},
	{
		"inputs": [
			{
				"internalType": "uint256",
				"name": "_newRequirement",
				"type": "uint256"
			}
		],
		"name": "setStakeRequirement",
		"outputs": [],
		"stateMutability": "nonpayable",
		"type": "function"
	},
	{
		"inputs": [
			{
				"internalType": "uint256",
				"name": "_amount",
				"type": "uint256"
			}
		],
		"name": "stake",
		"outputs": [],
		"stateMutability": "nonpayable",
		"type": "function"
	},
	{
		"inputs": [],
		"name": "stakeRequirement",
		"outputs": [
			{
				"internalType": "uint256",
				"name": "",
				"type": "uint256"
			}
		],
		"stateMutability": "view",
		"type": "function"
	},
	{
		"inputs": [],
		"name": "stakingToken",
		"outputs": [
			{
				"internalType": "contract IERC20",
				"name": "",
				"type": "address"
			}
		],
		"stateMutability": "view",
		"type": "function"
	},
	{
		"inputs": [
			{
				"internalType": "address",
				"name": "",
				"type": "address"
			}
		],
		"name": "stakes",
		"outputs": [
			{
				"internalType": "uint256",
				"name": "",
				"type": "uint256"
			}
		],
		"stateMutability": "view",
		"type": "function"
	},
	{
		"inputs": [],
		"name": "totalRewardsDistributed",
		"outputs": [
			{
				"internalType": "uint256",
				"name": "",
				"type": "uint256"
			}
		],
		"stateMutability": "view",
		"type": "function"
	},
	{
		"inputs": [],
		"name": "totalStaked",
		"outputs": [
			{
				"internalType": "uint256",
				"name": "",
				"type": "uint256"
			}
		],
		"stateMutability": "view",
		"type": "function"
	},
	{
		"inputs": [
			{
				"internalType": "address",
				"name": "newOwner",
				"type": "address"
			}
		],
		"name": "transferOwnership",
		"outputs": [],
		"stateMutability": "nonpayable",
		"type": "function"
	},
	{
		"inputs": [
			{
				"internalType": "uint256",
				"name": "_amount",
				"type": "uint256"
			}
		],
		"name": "unstake",
		"outputs": [],
		"stateMutability": "nonpayable",
		"type": "function"
	}
]; // Replace with the actual ABI

const stakingPortal = new web3.eth.Contract(contractABI, contractAddress);

// Example functions:

async function getAccount() {
  const accounts = await web3.eth.getAccounts();
  return accounts[0]; // Use the first account
}


async function stakeTokens(amount) {
    try {
        const account = await getAccount();
        // Convert to Wei if necessary (adjust decimals according to token)
        const amountInWei = web3.utils.toWei(amount.toString(), 'ether'); // Or 'gwei', 'mwei', etc. based on token decimals
        await stakingPortal.methods.stake(amountInWei).send({ from: account });
        console.log("Stake successful!");
    } catch (error) {
        console.error("Stake failed:", error);
    }
}

async function unstakeTokens(amount) {
    try {
        const account = await getAccount();
        const amountInWei = web3.utils.toWei(amount.toString(), 'ether');
        await stakingPortal.methods.unstake(amountInWei).send({ from: account });
        console.log("Unstake successful!");
    } catch (error) {
        console.error("Unstake failed:", error);
    }
}

async function claimRewards() {
    try {
        const account = await getAccount();
        await stakingPortal.methods.claimRewards().send({ from: account });
        console.log("Claim Rewards successful!");
    } catch (error) {
        console.error("Claim Rewards failed:", error);
    }
}

async function getStakedAmount(accountAddress) {
    try {
        //const account = await getAccount();
        const stakedAmount = await stakingPortal.methods.getStake(accountAddress).call();
        return web3.utils.fromWei(stakedAmount, 'ether'); //convert back from wei
    } catch (error) {
        console.error("Get Staked Amount failed:", error);
		return 0; // Handle errors gracefully, return a default value.
    }
}

async function getRewardsOwed(accountAddress) {
    try {
        const rewards = await stakingPortal.methods.getRewardsOwed(accountAddress).call();
        return web3.utils.fromWei(rewards, 'ether');
    } catch (error) {
        console.error("Get Rewards Owed failed:", error);
		return 0; // Handle errors gracefully.
    }
}

async function calculateEarnedRewards(accountAddress) {
  try {
    const earned = await stakingPortal.methods.earned(accountAddress).call();
    return web3.utils.fromWei(earned, 'ether');
  } catch (error) {
    console.error("Error calculating earned rewards:", error);
	return 0; // Handle errors.
  }
}


// Example usage (assuming you have buttons or form inputs in your HTML):

// Stake Button Click Handler
async function handleStakeButtonClick() {
    const stakeAmountInput = document.getElementById("stakeAmount"); // Assuming you have an input field with id "stakeAmount"
    const amount = parseFloat(stakeAmountInput.value);
    if (isNaN(amount) || amount <= 0) {
        alert("Please enter a valid stake amount.");
        return;
    }
    await stakeTokens(amount);
}

// Unstake Button Click Handler
async function handleUnstakeButtonClick() {
    const unstakeAmountInput = document.getElementById("unstakeAmount"); // Assuming you have an input field with id "unstakeAmount"
    const amount = parseFloat(unstakeAmountInput.value);
    if (isNaN(amount) || amount <= 0) {
        alert("Please enter a valid unstake amount.");
        return;
    }
    await unstakeTokens(amount);
}

// Claim Rewards Button Click Handler
async function handleClaimRewardsButtonClick() {
    await claimRewards();
}

// Display Staked Amount
async function displayStakedAmount(accountAddress) {
  const stakedAmountDisplay = document.getElementById("stakedAmountDisplay"); // Element to display stake
  const stakedAmount = await getStakedAmount(accountAddress);
  stakedAmountDisplay.textContent = `Staked Amount: ${stakedAmount} Token`;
}

// Display Rewards Owed
async function displayRewardsOwed(accountAddress) {
  const rewardsOwedDisplay = document.getElementById("rewardsOwedDisplay"); // Element to display rewards
  const rewardsOwed = await getRewardsOwed(accountAddress);
  rewardsOwedDisplay.textContent = `Rewards Owed: ${rewardsOwed} Token`;
}

// Display Earned Rewards
async function displayEarnedRewards(accountAddress) {
  const earnedRewardsDisplay = document.getElementById("earnedRewardsDisplay"); // element to display earned rewards
  const earnedRewards = await calculateEarnedRewards(accountAddress);
  earnedRewardsDisplay.textContent = `Earned Rewards: ${earnedRewards} Token`;
}

// Initialization function
async function init() {
  try {
    const account = await getAccount();
    displayStakedAmount(account);
    displayRewardsOwed(account);
	displayEarnedRewards(account);
  } catch (error) {
    console.error("Initialization error:", error);
  }
}

// Call init when the page loads
window.addEventListener('load', init);


```

**Explanation and Improvements:**

1.  **Solidity Contract (`CrossChainStakingPortal.sol`):**

    *   **`pragma solidity ^0.8.0;`**: Specifies the Solidity compiler version.  Use a version 0.8.0 or higher to prevent integer overflow errors.

    *   **`import "@openzeppelin/contracts/token/ERC20/IERC20.sol";`**: Imports the IERC20 interface from OpenZeppelin.  This allows you to interact with any ERC20-compliant token.  It's essential for working with tokens in Solidity.  You must install OpenZeppelin contracts using npm or yarn: `npm install @openzeppelin/contracts` or `yarn add @openzeppelin/contracts`.

    *   **`import "@openzeppelin/contracts/access/Ownable.sol";`**: Imports the `Ownable` contract from OpenZeppelin.  This provides basic access control, allowing only the contract owner to call certain functions.

    *   **`IERC20 public stakingToken;`**:  A public variable to store the address of the ERC20 token contract being staked.  Using the `IERC20` interface allows you to call functions like `transfer` and `transferFrom`.

    *   **`uint256 public stakeRequirement;`**:  A public variable that defines the minimum amount required to stake.

    *   **`uint256 public totalStaked;`**: Keeps track of the total amount of tokens staked in the contract.  This is important for calculating rewards fairly.

    *   **`mapping(address => uint256) public stakes;`**: A mapping that stores the stake amount for each address.  This allows you to quickly look up how much a user has staked.

    *   **`mapping(address => uint256) public rewardsOwed;`**:  A mapping to store the amount of rewards owed to each address.  This is crucial for accumulating rewards over time.

    *   **`uint256 public rewardRatePerBlock;`**:  The amount of rewards distributed per block, per staked token (scaled by `1e18`). The smaller you make the `rewardRatePerBlock` the more tokens will be required to stake.
        It will be required to be increased if nobody deposits any tokens.

    *   **`uint256 public lastRewardBlock;`**:  The block number when the last reward distribution occurred.  This is used to calculate how many blocks have passed since the last update.

    *   **Events**: Events are emitted to log important actions.  They are crucial for off-chain monitoring and debugging.  Events also make it easier for front-end applications to react to changes in the contract's state.

    *   **`constructor(address _stakingTokenAddress, uint256 _stakeRequirement, uint256 _rewardRatePerBlock) Ownable()`**:  The constructor initializes the contract with the address of the staking token, the stake requirement, and the reward rate. The `Ownable()` call initializes the `Ownable` contract and sets the deployer as the owner.

    *   **`modifier onlyIfStaked()`**: A custom modifier that checks if the sender has an active stake. This prevents users who haven't staked from calling functions like `unstake` and `claimRewards`.

    *   **`setStakeRequirement()` and `setRewardRate()`**:  Admin functions to change the stake requirement and reward rate.  These functions are protected by the `onlyOwner` modifier.

    *   **`recoverERC20()`**: An admin function to recover accidentally sent ERC20 tokens.  This is a safety feature to prevent tokens from being permanently lost.  Crucially, it prevents recovering the staking token.

    *   **`stake()`**: Allows users to stake tokens. It checks if the stake amount meets the requirement, transfers tokens from the user to the contract, updates the user's stake and total staked amount, and emits a `Stake` event. `updateRewards` must be called before updating the stakes map to make sure rewards are calculated.

    *   **`unstake()`**: Allows users to unstake tokens. It checks if the unstake amount is valid, updates the user's stake and total staked amount, transfers tokens from the contract to the user, and emits an `Unstake` event.

    *   **`claimRewards()`**: Allows users to claim their accumulated rewards.  It transfers the rewards from the contract to the user and resets the user's rewards owed.  `updateRewards` ensures that the current earned rewards are calculated before being claimed.

    *   **`updateRewards()`**: Calculates and distributes pending rewards based on the time since the last distribution and the current total staked amount.

    *   **`calculateRewards()`**: This function calculates how much rewards has been earned since the last reward distribution.

    *   **`earned()`**: This view function calculates how much reward a given user has earned.

    *   **`getStake()` and `getRewardsOwed()`**:  View functions to retrieve a user's stake amount and rewards owed.

    *   **Important Security Considerations:**

        *   **Re-entrancy Attack:** This is a major vulnerability in smart contracts.  A malicious contract could call back into your contract during the `transfer` function in `claimRewards` or `unstake`, potentially draining the contract. To mitigate this, implement a re-entrancy guard using the `@openzeppelin/contracts/security/ReentrancyGuard.sol` contract.  This prevents recursive calls.  Wrap your functions with the `nonReentrant` modifier.

        *   **Integer Overflow/Underflow:**  Solidity versions before 0.8.0 are vulnerable to integer overflows and underflows. Use Solidity 0.8.0 or higher, which includes built-in overflow/underflow checks.

        *   **Denial of Service (DoS):** If the reward rate is very high, the `claimRewards` function might require a large amount of gas, potentially exceeding the block gas limit.  This could prevent users from claiming their rewards.  Consider limiting the maximum amount of rewards that can be claimed in a single transaction.

        *   **Front-Running:**  A malicious actor could monitor the blockchain for pending `stake` or `unstake` transactions and front-run them to their advantage.  Consider using commit-reveal schemes or other mitigation techniques to prevent front-running.

    *   **Gas Optimization:**

        *   **Use `unchecked` blocks:**  In Solidity 0.8.0+, you can use `unchecked` blocks to disable overflow/underflow checks in situations where you are certain that overflows/underflows cannot occur.  This can save gas. Be very careful when using `unchecked` blocks.

        *   **Storage vs. Memory:**  Avoid unnecessary storage writes.  Storage is much more expensive than memory.  Cache values in memory when possible.

        *   **Packed Storage Variables:**  Declare variables in the order they are used, and try to group smaller variables together to pack them into a single storage slot.

        *   **Short Strings:** Use `bytesX` where X is the length of the string.  This will prevent padding of the `string` type which uses 32 bytes to store the length of the string.

2.  **JavaScript (Web3.js) (`script.js`):**

    *   **`web3`**: This assumes you have Web3 injected into your browser (usually via MetaMask). If not, you'll need to initialize Web3 with a provider (e.g., Infura, Alchemy).

    *   **`contractAddress` and `contractABI`**:  Replace these with your actual contract address and ABI.  The ABI is essential for Web3.js to understand the contract's functions and events.  You can get the ABI from the Solidity compiler output.

    *   **`stakingPortal = new web3.eth.Contract(contractABI, contractAddress);`**:  Creates a Web3.js contract instance, allowing you to interact with the contract.

    *   **`getAccount()`**: Asynchronously retrieves the user's Ethereum account from MetaMask.

    *   **`stakeTokens(amount)`**:  Calls the `stake` function on the contract.  It converts the amount to Wei (the smallest unit of Ether) before sending the transaction.

    *   **`unstakeTokens(amount)`**: Calls the `unstake` function on the contract, similarly converting the amount to Wei.

    *   **`claimRewards()`**:  Calls the `claimRewards` function on the contract.

    *   **`getStakedAmount(accountAddress)`**: Calls the `getStake` function on the contract and converts the result from Wei back to Ether for display.

    *   **`getRewardsOwed(accountAddress)`**:  Calls the `getRewardsOwed` function and converts the result from Wei to Ether.

	*   **`calculateEarnedRewards(accountAddress)`**: Calls the `earned` function and converts the result from Wei to Ether.

    *   **Error Handling**:  The JavaScript functions include `try...catch` blocks to handle potential errors during contract interactions.

    *   **UI Interaction**: The example includes functions like `handleStakeButtonClick()`, `handleUnstakeButtonClick()`, and `handleClaimRewardsButtonClick()` that would be triggered by user interactions with HTML buttons or forms. It also includes `displayStakedAmount`, `displayRewardsOwed`, and `displayEarnedRewards` to update the user interface with the current state of the contract.  It shows how to get values from input fields and display them on the page.

    *   **`init()`**: This function initializes the app by getting the current account and displaying the staked amount and rewards owed. This ensures that the UI is up-to-date when the page loads.

    *   **`window.addEventListener('load', init);`**:  This ensures that the `init()` function is called when the page finishes loading.

3. **Cross-Chain Considerations (Conceptual)**
    This is where the example stops short.  True cross-chain functionality is very complex. Here's what you'd need to consider:

    *   **Bridging:** You need a mechanism to move the staking token between chains. This typically involves:
        *   **Locking:** Locking tokens on the origin chain.
        *   **Minting/Releasing:** Minting/releasing representative tokens on the destination chain.
        *   **Oracles:**  Oracles verify that the lock/mint/burn events have occurred on each chain. Chainlink is often used.
    *   **Interoperability Standards:** Standards such as LayerZero or IBC (Cosmos) aim to make cross-chain communication easier.
    *   **Complexity:** Cross-chain development significantly increases the complexity of your application. You're dealing with multiple chains, potential security vulnerabilities, and increased gas costs.

4.  **Example HTML (Conceptual):**

```html
<!DOCTYPE html>
<html>
<head>
    <title>Cross-Chain Staking Portal</title>
    <script src="https://cdn.jsdelivr.net/npm/web3@1.6.0/dist/web3.min.js"></script>
    <script src="script.js"></script>
</head>
<body>
    <h1>Cross-Chain Staking Portal</h1>

    <div>
        <label for="stakeAmount">Stake Amount:</label>
        <input type="number" id="stakeAmount">
        <button onclick="handleStakeButtonClick()">Stake</button>
    </div>

    <div>
        <label for="unstakeAmount">Unstake Amount:</label>
        <input type="number" id="unstakeAmount">
        <button onclick="handleUnstakeButtonClick()">Unstake</button>
    </div>

    <div>
        <button onclick="handleClaimRewardsButtonClick()">Claim Rewards</button>
    </div>

    <p id="stakedAmountDisplay">Staked Amount: </p>
    <p id="rewardsOwedDisplay">Rewards Owed: </p>
	<p id="earnedRewardsDisplay">Earned Rewards: </p>
</body>
</html>
```

**Important Next Steps:**

*   **Thorough Testing:**  Write extensive unit tests to cover all possible scenarios, including edge cases and potential vulnerabilities.  Use tools like Hardhat or Truffle for testing.
*   **Security Audits:**  Have your contract audited by a reputable security firm.
*   **Gas Optimization:**  Profile your contract's gas usage and optimize for efficiency.
*   **User Experience:**  Design a user-friendly interface that makes it easy for users to stake, unstake, and claim rewards.
*   **Documentation:**  Provide clear and comprehensive documentation for your contract and application.

This thoroughly revised example provides a good starting point for building a cross-chain staking portal. Remember to prioritize security, gas optimization, and a good user experience.  Cross-chain functionality adds significant complexity, so carefully consider the architecture and security implications before implementation.