HTML Checking for Large Sites

A button element, or an element with the role=button attribute, is not allowed to be nested inside an <a> element.

An <a> element cannot contain a descendant element with the attribute tabindex.

Learn more:

• MDN: tabindex

The value contact is not a valid option for the autocomplete attribute on an <input> element.

Learn more:

• HTML Spec: autocomplete

All HTML elements may have the hidden boolean attribute set. When specified on an element, it indicates that the element is not yet, or is no longer, relevant, so browsers won’t render it.

Boolean attributes don’t accept values, its presence represents the true value and its absence represents the false value.

<!-- This is invalid because the hidden attribute should not have a value set -->
<div hidden="false"></div>

<!-- The correct way to hide a div is like this -->
<div hidden>This will be hidden</div>

<!-- And to show the element, we just don't hide it -->
<div>This won't be hidden</div>

Read more:

• HTML Spec: the hidden attribute
• HTML Spec: boolean attributes

An <a> element has been found with an invalid href attribute, containing more than one # adjacent character.

The # is used to separate the fragment part of an URI (typically used to indicate a section within a document). For example, this is a valid link to a URI containing a fragment:

<a href="https://example.com/faqs#pricing">pricing</a>

The next example is invalid because it contains two adjacent # characters, so that the fragment part would be #pricing instead of pricing:

<a href="https://example.com/faqs##pricing">pricing</a>

Learn more:

• Wikipedia: URI fragment

The href attribute of an <a> element contains an invalid character, that should be properly encoded as a URI percent-encoded character.

Learn more:

• Wikipedia: percent-encoding

The href attribute on the link element must not be empty.

The src attribute on an <img> element contains an invalid character, that should be properly encoded as a URI percent-encoded character.

Learn more:

• Wikipedia: percent-encoding

The target attribute on <a> elements can’t be blank.

This attribute defines the browsing context for links, that is, where should the linked documents be opened. This was used extensively on the now deprecated <frame> element, so you could give the name of the frame to open the document in, but is now more used to force links to open in a separate tab or window using target="_blank". Another option is using a name, so the new browsing context can be referred to on subsequent clicks on links with the same target.

For example, this will force the links to open on a new tab:

<a href="https://example.com" target="_blank">will open a blank tab</a>

Learn more:

• HTML Living Standard: Browsing context names

The href attribute on an a tag expects a valid URL, but only http:// was found.

The href attribute on an <a> link contains a space character, which is not allowed. If you’re trying to link to a phone URL, review the href attribute to remove unallowed characters, as in this example:

<!-- Invalid as it contains a space character -->
<a href="tel: +123456789">call me</a>

<!-- Valid -->
<a href="tel:+123456789">call me</a>

Source:

• IETF: tel URI for Telephone Numbers

Space characters are not allowed in href attributes. Instead, they should be converted to %20. In this example, the first line is invalid and the second is valid:

<a href="https://example.com#some term">invalid</a>
<a href="https://example.com#some%20term">valid</a>

The href attribute on an element <a> contains a character that is not allowed, and should be encoded.

Some typical examples include the pipe character | that should be replaced by its encoded alternative %7C , and the left square bracket [ that needs to be encoded as %5B.

The href attribute on an <a> tag contains an space, which is not allowed. Consider replacing space characters with “%20”.

The at symbol (@) should be percent-encoded as %40 in order to include it at an href attribute.

The href attribute on an element <link> contains a character that is not allowed, and should be encoded.

Some typical examples include the pipe character | that should be replaced by its encoded alternative %7C , and the left square bracket [ that needs to be encoded as %5B.

A <pattern> element has been found with an invalid ID. Check the format of the ID and ensure it does not start with a digit, full stop (.) or hyphen (-).

The <pattern> element is used within <svg> elements, which use XML 1.0 syntax. That syntax specifies that valid IDs only include designated characters (letters, digits, and a few punctuation marks), and do not start with a digit, a full stop (.) character, or a hyphen-minus (-) character.

Learn more:

• MDN: pattern element
• MDN: SVG id

The media attribute on a <link> element has not been recognized.

This attribute specified what media the linked resource is optimized for. As an example, the following will link a general stylesheet, and a specific one for printing:

<head>
  <link rel="stylesheet" type="text/css" href="general.css">
  <link rel="stylesheet" type="text/css" href="print.css" media="print">
</head>

Valid values for this attribute include:

• all. Default, used for all media.
• print. Used for printers and print previews.
• screen. Used for computer, tablets or smartphone screens.

Learn more:

• W3: Media Type

The value used in the target property of an <a> element has been identified as a keyword because it starts with an underscore _, but it’s not a valid keyword.

Values starting with an underscore for the target property are used for reserved keywords like _blank, _self, _parent or _top.

Learn more:

• HTML Spec: links created by a elements
• HTML Spec: valid browsing content or keyword

The value provided on the type attribute of an a element is not a valid MIME type.

The type attribute expects a MIME type that hints at the linked URL’s format.

Read more:

• MDN: type attribute on a
• MDN: common MIME types

The type attribute on <a> elements, when present, gives a hint on the MIME type of the linked resource, for example:

<a href="application/pdf" src="book.pdf">Read our book</a>
<a href="image/jpeg" src="photo.jpeg">See a photo</a>

That is, we’re talking about the type of the linked resource, not the type of the <a> element, as it’s sometimes misunderstood. The following example is invalid because button is not a valid MIME type.

<a href="/order.php" type="button">Submit</a>

Learn more:

• HTML Spec: the type attribute

The value used to define the type of a link is not valid. You’re probably using a URL instead of a valid type.

Example of a valid type:

<link rel="icon" type="image/png" href="favicon.png">

Read more:

• HTML Spec: link types

The <a> element requires either a href attribute, or a role attribute.

Learn more:

• HTML Spec: the a element

An </a> end tag has been found to violate nesting rules. <a> tags can’t include other <a> tags inside. Most probable cause is an unclosed <a> tag, like in this example:

<a href="one.html">Page 1
<a href="two.html">Page 2</a>

Attributes in HTML elements need to be separated by space, in this example the first line is invalid and the second one is valid:

<a href="page.php"class="big">link</a>

<a href="page.php" class="big">link</a>

An <a> tag can’t include other <a> tags inside. Most probable cause is an unclosed <a> tag, like in this example:

<a href="one.html">Page 1
<a href="two.html">Page 2</a>

An <a> element has been found in an invalid place within a <table> element.

For example, the following code would cause this issue:

<table>
  <tr>
    <a href="#">link</a>
  </tr>
</table>

Instead, the <a> element should be inside a <td> element, as a <tr> can’t hold content directly:

<table>
  <tr>
    <td>
      <a href="#">link</a>
    </td>
  </tr>
</table>

A button element, or an element with the role=button attribute, is not allowed to be nested inside an <a> element.

A <label> tag can’t be used inside an <a> tag. Consider using other tags like <span>.

Links created with the <a> element no longer accept a shape attribute. In order to define image maps, use the <area> element instead.

Learn more:

• MDN: area element