HTML Guide
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>Related W3C validator issues
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>A <button> tag can’t include other <button> tags inside. Most probable cause is an unclosed <button> tag, like in this example:
<button>Submit
<button>Cancel</button>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 <textarea> tag can’t be used inside an <a> tag.
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.
A <p> element cannot be placed inside a <noscript> tag within the <head> section.
According to the HTML specification, the <head> element must only contain metadata, such as <title>, <meta>, <link>, <script>, and <style>. The <noscript> element is allowed in <head>, but it must only contain elements that are valid in head, not flow content like <p>. The <p> (paragraph) tag is flow content meant for the <body>. For fallback content in <head>, only metadata elements are allowed.
Incorrect example:
<!DOCTYPE html>
<html lang="en">
<head>
<title>Example</title>
<noscript>
<p>JavaScript is disabled.</p>
</noscript>
</head>
<body>
</body>
</html>Correct approaches:
-
Remove the <p> from <noscript> in <head>:
- If you must include fallback styles or links in case JavaScript is disabled, use only metadata tags.
<!DOCTYPE html>
<html lang="en">
<head>
<title>Example</title>
<noscript>
<link rel="stylesheet" href="no-js.css">
</noscript>
</head>
<body>
</body>
</html>-
Place textual fallback content in the <body> instead:
- Moving the <noscript> block with flow content (such as <p>) to the body ensures validity.
<!DOCTYPE html>
<html lang="en">
<head>
<title>Example</title>
</head>
<body>
<noscript>
<p>JavaScript is disabled.</p>
</noscript>
</body>
</html>Remember: Do not use <p> (or any flow content) in <noscript> inside <head>. Use such content only in the body.
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.