HTML Validation

Bad value X for attribute “name” on element “iframe”: Browsing context name started with the underscore.

About This HTML Issue

The value of the name attribute on an <iframe> should not start with an underscore (_).

Browsing context names that begin with an underscore are reserved keywords in HTML, like _blank, _self, _parent, and _top. Using these reserved names or any custom name starting with an underscore for the name attribute of an <iframe> can lead to unexpected behavior and is considered invalid HTML.

Here’s how to fix the issue:

Problematic Code

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

Solution

To resolve this issue, you should use a valid value for the name attribute that does not start with an underscore.

Corrected Code

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

Steps:

Identify the iframe element with the invalid name attribute value that starts with an underscore.
Replace the name value with a valid identifier that does not start with _. Use letters, numbers, hyphens (-), and underscores (_) (but not at the beginning).

Learn more:

→ MDN: iframe name
→ MDN: browsing context

Last reviewed: September 13, 2024

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Reading time

1 min read

Category

HTML Validation

Engine

W3C Validator

Last reviewed

September 13, 2024

Resources

Related W3C Validator Issues (84)

Bad value “” for attribute “name” on element “a”: An ID must not be the empty string.

Empty name attributes on <a> are invalid; an anchor’s name/id must be a non-empty unique identifier.

Historically, the name attribute on <a> was used to create fragment targets (e.g., <a name="top"></a>). In HTML5/living standard, fragment navigation should use the id attribute instead. The name attribute on <a> is obsolete for this purpose, and an empty string is never allowed for identifiers.

Use id with a non-empty value to create link targets, and point to them with href="#idValue". If you keep name for legacy reasons, it must still be non-empty and match the HTML syntax for valid IDs. Avoid adding empty attributes like name="" or id="". Also ensure uniqueness of each id in the document.

HTML Examples

Example causing the validator error

<!doctype html>
<html lang="en">
  <head>
    <title>Anchor name error</title>
  </head>
  <body>
    <a name=""></a>
    <a href="#"></a>
  </body>
</html>

Corrected example using id (recommended)

<!doctype html>
<html lang="en">
  <head>
    <title>Anchor id fix</title>
  </head>
  <body>
    <a id="top"></a>
    <a href="#top">Back to top</a>
  </body>
</html>

Bad value “” for attribute “name” on element “input”: Must not be empty.

Bad value for attribute “src” on element “iframe”: Illegal character in query: “[” is not allowed.

Illegal character “[” in the iframe src URL requires percent-encoding or removal.

The iframe element’s src must be a valid URL. According to URL syntax, characters like [ and ] are not allowed in the query unless percent-encoded. If your src contains array-like parameters (e.g., filters[category]=news), encode reserved characters: [ becomes %5B and ] becomes %5D. Avoid spaces and encode other reserved characters as needed. Alternatively, adjust the server to accept dot or bracketless notation (e.g., filters.category=news or filters_category=news) so the URL stays valid without encoding.

HTML Examples

Example causing the validator error

<!DOCTYPE html>
<html lang="en">
<head>
  <title>Iframe URL Error</title>
</head>
<body>
<!-- [ and ] are illegal in URLs unless encoded -->

  <iframe src="https://example.com/embed?filters[category]=news&filters[tags]=web"></iframe>
</body>
</html>

Fixed example with percent-encoding

<!DOCTYPE html>
<html lang="en">
<head>
  <title>Iframe URL Fixed</title>
</head>
<body>
<!-- Encode [ as %5B and ] as %5D -->

  <iframe src="https://example.com/embed?filters%5Bcategory%5D=news&filters%5Btags%5D=web"></iframe>
</body>
</html>

Attribute “name” not allowed on element “meta” at this point.

meta
name

A <meta> tag has been found that is either malformed, or in a bad place within the document. Check its attributes and context.

For example, the following HTML contains a valid <meta> tag that is raising an issue because of bad context, caused by an <img> tag that shouldn’t be there:

<!DOCTYPE html>
<html lang="">
  <head>
    <title>Test</title>
    <img src="photo.jpg" alt="A smiling cat" />
    <meta name="description" content="Description of this page" />
  </head>
  <body>
    <p>Some content</p>
  </body>
</html>

If we fix that document and move the <img> tag within the body, the issue raised about <meta> disappears because it’s now in a valid context:

<!DOCTYPE html>
<html lang="">
  <head>
    <title>Test</title>
    <meta name="description" content="Description of this page" />
  </head>
  <body>
    <p>Some content</p>
    <img src="photo.jpg" alt="A smiling cat" />
  </body>
</html>

Attribute “seamless” not allowed on element “iframe” at this point.

Bad start tag in “iframe” in “noscript” in “head”.

There is an iframe tag inside a noscript tag that is itself inside the head section of the HTML document. This is not allowed because an iframe cannot be nested inside the head section.

To fix this issue, you may move the noscript section that contains the iframe tag outside of the head section, and ensure that it is placed within the body section of the HTML document.

For example, this is invalid HTML because the head section cannot contain iframe elements:

<!DOCTYPE html>
<html lang="en">
  <head>
    <title>My webpage</title>
    <noscript>
      <p>Please enable JavaScript to view this website</p>
      <iframe src="https://example.com/"></iframe>
    </noscript>
<!-- Other meta tags and styles go here -->

  </head>
  <body>
<!-- Rest of your webpage content goes here -->

  </body>
</html>

Moving the noscript inside the body section fixes the issue, as that’s where iframe elements belong:

<!DOCTYPE html>
<html lang="en">
  <head>
    <title>My webpage</title>
<!-- Other meta tags and styles go here -->

  </head>
  <body>
    <noscript>
      <p>Please enable JavaScript to view this website</p>
      <iframe src="https://example.com/"></iframe>
    </noscript>
<!-- Rest of your webpage content goes here -->

  </body>
</html>

Bad value “allow-storage-access-by-user-activation” for attribute “sandbox” on element “iframe”: The string “allow-storage-access-by-user-activation” is not a valid keyword.

Bad value (true|false) for attribute “itemscope” on element “html”.

The itemscope attribute is a boolean attribute in HTML5, which means it does not take any values. Adding any value (such as true or false) will cause an error. When using boolean attributes, they should either be present or absent. If an attribute like itemscope is present, it is considered true.

Here’s how to correct the error:

Incorrect Usage:

<div itemscope="true">

Correct Usage:

<div itemscope>

Explanation:

Simply including the itemscope attribute without any value is the correct way to use it.
If you don’t want to use the itemscope attribute, just remove it from the tag.

Bad value “callto:X” for attribute “href” on element “a”: Illegal character in scheme data: space is not allowed.

Spaces are not permitted in the href value for phone links; the phone number must be a continuous string without spaces or slashes.

The href attribute of an anchor (<a>) element defines the link’s destination. For phone numbers, the proper URI scheme is tel:, not callto:. According to the HTML standard and the WHATWG Living Standard, the phone number should contain only digits and may use plus (+) or hyphen (-) characters for formatting, but it should not include spaces or slashes.

Incorrect HTML:

<a href="callto:07142/ 12 34 5">Call us</a>

Correct HTML:

<a href="tel:0714212345">Call us</a>

With country code and optional formatting:

<a href="tel:+49714212345">Call us</a>

For best compatibility and validation, always use the tel: scheme and ensure the phone number string contains only allowed characters.

Bad value “contact” for attribute “autocomplete” on element “input”: The string “contact” is not a valid autofill field name.

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