HTML Guides for underscore

In HTML, the name attribute on an <iframe> defines a browsing context name. This name can be referenced by other elements — for example, a link with target="my-frame" will open its URL inside the <iframe> whose name is "my-frame". The HTML specification reserves all browsing context names that start with an underscore for special keywords:

• _self — the current browsing context
• _blank — a new browsing context
• _parent — the parent browsing context
• _top — the topmost browsing context

Because the underscore prefix is reserved for these (and potentially future) keywords, the spec requires that any custom browsing context name must not begin with _. Setting name="_example" or name="_myFrame" on an <iframe> is invalid HTML, even though those exact strings aren’t currently defined keywords. Browsers may handle these inconsistently — some might ignore the name entirely, while others could treat it as one of the reserved keywords, leading to unexpected navigation behavior.

This matters for several reasons:

• Standards compliance: The WHATWG HTML living standard explicitly states that a valid browsing context name must not start with an underscore character.
• Predictable behavior: Using a reserved prefix can cause links or forms targeting that <iframe> to navigate in unintended ways (e.g., opening in a new tab instead of within the frame).
• Future-proofing: New underscore-prefixed keywords could be added to the spec, which might break pages that use custom names starting with _.

To fix the issue, simply rename the name attribute value so it doesn’t start with an underscore. You can use underscores elsewhere in the name — just not as the first character.

Examples

❌ Invalid: name starts with an underscore

<iframe src="https://example.com" name="_example"></iframe>

<a href="https://example.com/page" target="_example">Open in frame</a>

The name _example starts with an underscore, which makes it invalid. A browser might interpret _example unpredictably or ignore the name entirely when used as a target.

✅ Fixed: underscore removed from the start

<iframe src="https://example.com" name="example"></iframe>

<a href="https://example.com/page" target="example">Open in frame</a>

✅ Fixed: underscore used elsewhere in the name

<iframe src="https://example.com" name="my_example"></iframe>

<a href="https://example.com/page" target="my_example">Open in frame</a>

Underscores are perfectly fine as long as they aren’t the first character.

❌ Invalid: using a reserved keyword as a frame name

<iframe src="https://example.com" name="_blank"></iframe>

Using _blank as an <iframe> name is also invalid because it’s a reserved browsing context keyword. A link targeting _blank would open in a new window/tab rather than inside this <iframe>, which is almost certainly not what you intended.

✅ Fixed: descriptive name without underscore prefix

<iframe src="https://example.com" name="content-frame"></iframe>

<a href="https://example.com/page" target="content-frame">Open in frame</a>

How to fix

1. Find every <iframe> element whose name attribute starts with _.
2. Rename each one by removing the leading underscore or replacing it with a letter or other valid character.
3. Update any target attributes on <a>, <form>, or <base> elements that reference the old name so they match the new name.
4. Re-validate your HTML to confirm the issue is resolved.