HTML XHTML

HTML vs. XHTML: Strict XML Rules vs. Modern HTML5

XHTML (EXtensible HyperText Markup Language) is a stricter, XML-compliant reformulation of HTML4 introduced in 2000. While modern web development uses HTML5, knowing XHTML's strict rules explains why many codebases still quote attributes, keep tags lowercase, and sometimes write self-closing slashes.

Key Differences Between HTML5 and XHTML

XML parsers halt completely on minor syntax mistakes, whereas HTML5 defines standardized error-recovery rules. XHTML enforces four strict formatting requirements:

1. All Elements Must Close: Non-void elements need closing tags, and void elements like br or img must include a trailing slash (/>).
2. All Tags and Attributes Must Be Lowercase: Uppercase tags or attribute names trigger fatal XML errors in XHTML.
3. Every Attribute Value Must Be Quoted: Writing tabindex=0 fails; you must write tabindex="0".
4. No Minimized Boolean Attributes: Write disabled="disabled" or checked="checked" instead of bare disabled.

<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Modern HTML5 vs Strict XML Style</title>
</head>
<body>
    <!-- Modern HTML5 syntax -->
    <form>
        <label for="api-key">API Key</label>
        <input id="api-key" type="text" value="readonly-demo" disabled>
        <br>
        <button type="submit">Save</button>
    </form>
</body>
</html>

JSX and SVG Echoes Today

Even though standalone XHTML pages are rare today, React JSX and inline SVG files still borrow XML strictness—meaning every tag in JSX must close with a closing tag or self-closing /> slash, and mismatched nesting causes an immediate compile error.

Common Mistakes to Avoid

• Adding an XHTML DOCTYPE to new HTML5 projects: Always open modern web pages with the short HTML5 DOCTYPE html declaration.
• Using unclosed HTML5 void tags inside React JSX: Remember that .jsx and .tsx components require self-closing slashes on void elements like input and br.
• Serving XHTML as application/xhtml+xml without testing: One stray unescaped & in user content crashes the whole page with an XML parsing error.

Frequently Asked Questions

Should I learn XHTML or HTML5 today?

Learn HTML5. It is the living standard supported by every browser, while XHTML remains mainly of historical interest and as inspiration for JSX syntax.

Are self-closing slashes on br tags valid in HTML5?

Yes, HTML5 parsers ignore the optional trailing slash on void elements like br and hr, though omitting it is cleaner in pure HTML.