Finding a way to successfully run HTML and CSS on WordPress

<footer>
    <p>Created by <a href="https://fireplaceadviser.com" target="_blank">fireplaceadviser.com</a></p>
</footer>

<script>
    function calculateFireplaceSize() {
        const length = document.getElementById('roomLength').value;
        const width = document.getElementById('roomWidth').value;
        const height = document.getElementById('roomHeight').value;

        if (length && width && height) {
            const roomVolume = length * width * height;
            const recommendedFireplaceSize = roomVolume * 0.034; // Example calculation
            document.getElementById('result').innerText = 
                `Recommended Fireplace Size: ${recommendedFireplaceSize.toFixed(2)} cubic feet per hour`;
        } else {
            document.getElementById('result').innerText = 'Please fill in all fields.';
        }
    }
</script>