HTML5
Are HTML5 Tags Case Sensitive?
A common interview question is whether HTML5 tags are case sensitive. The short answer is no: HTML5 tag and attribute names are case-insensitive, meaning the browser treats <DIV>, <Div>, and <div> as exactly the same element. However, writing tags in lowercase is the strongly recommended best practice.
Direct Answer
HTML5 tag names are NOT case sensitive. The parser normalises element and attribute names to lowercase internally, so <H1>Hello</H1> and <h1>Hello</h1> produce the identical result. This is different from XHTML, which required lowercase, and from languages like XML and JavaScript, which are strictly case sensitive.
Runnable Example
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<title>Case Sensitivity Demo</title>
</head>
<body>
<H1>Uppercase H1 tag</H1>
<h1>Lowercase h1 tag</h1>
<P>Both headings above look and behave the same.</P>
<DiV>Even mixed-case DiV works fine.</DiV>
</body>
</html>What Actually IS Case Sensitive?
| Part of HTML | Case Sensitive? |
|---|---|
| Tag names (<div>, <p>) | No |
| Attribute names (class, href) | No |
| Attribute values in general | Depends (usually significant) |
| id and class values | Yes — must match exactly in CSS/JS |
| File paths / URLs in src, href | Yes on most servers |
While tag names are case-insensitive, the VALUES of id and class attributes ARE case sensitive. <div id="Main"> and #main in CSS will not match.
Why Use Lowercase?
- Lowercase is the universal community and style-guide standard.
- It improves readability and consistency across a codebase.
- It is required if you ever serve documents as XHTML/XML.
- Linters and formatters (like Prettier) expect and enforce it.
Key Takeaways
- HTML5 tag and attribute names are case-insensitive.
- Always write tags in lowercase as best practice.
- id and class VALUES remain case sensitive.