HTML Guide

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>

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>

The <iframe> HTML element represents a nested browsing context, embedding another HTML page into the current one.

As the iframe is a container that holds an embedded HTML page, it cannot be nested inside an a tag.

An <input> tag can’t be used inside an <a> tag.

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

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.

The <textarea> element does not have a type attribute.

The HTML <textarea> element represents a multi-line plain-text editing control, and is useful when you want to allow users to enter a sizeable amount of free-form text, for example a comment on a review or feedback form.

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.

Empty aria-controls attribute values are invalid; the attribute must reference the id of one or more elements.

The aria-controls attribute is used to indicate that the element controls the referenced element(s) by their id. According to the ARIA specification and W3C HTML standard, the attribute must contain at least one valid id value, and cannot be an empty string. Leaving aria-controls="" triggers a validation error.

Correct Usage:

• Assign an id to the element being controlled.
• Set the aria-controls attribute to match that id.
• Remove aria-controls entirely if not necessary.

Incorrect Example:

<a href="#" aria-controls="">Toggle</a>

Corrected Example:

<div id="details">Some details...</div>
<a href="#" aria-controls="details">Toggle</a>

If no element is being controlled:

<a href="#">Toggle</a>