Top 10 HTML Issues
Web developers worldwide have found 335 million HTML issues on 10 million checked web pages.
Here are the most common issues detected by Rocket Validator.
Last update: Friday, January 17, 2025
1. Trailing slash on void elements has no effect and interacts badly with unquoted attribute values. 45.55%
Void elements, like area, base, br, col, embed, hr, img, input, link, meta, source, track, and wbr are self-closing and don’t need a trailing slash /, which should be avoided as that can interfere with unquoted attribute values.
2. The first occurrence of ID “X” was here. 12.28%
Element IDs in an HTML document must be unique. The HTML validator is indicating the first occurrence of an ID that is repeated. Check the details for that issue to see web pages affected, and the elements within them, to fix that ID repetition.
3. Duplicate ID “X”. 12.28%
The id attribute is used to identify a single element within a document, and is required to be unique. Check the document for repeated IDs.
4. The “type” attribute is unnecessary for JavaScript resources. 8.40%
The default type for <script> tags is JavaScript, so you don’t need to include the type for JS resources.
5. No space between attributes. 5.93%
Attributes in HTML elements need to be separated by space.
6. Attribute “X” not allowed on element “Y” at this point. 5.48%
An invalid attribute has been found on an element. Check the affected tag to ensure attributes are well-formed, and if they are you can consider using custom data attributes.
7. Element “X” not allowed as child of element “Y” in this context. 4.26%
The element X is not allowed as a child element of Y. For example, a <ul> element cannot have a <div> child element.
8. Malformed byte sequence. 2.59%
The document could not be properly parsed due to malformed characters. Check the document encoding.
9. Attribute “X” is not serializable as XML 1.0. 1.64%
An attribute could not be parsed from the HTML input, probably due to a typo. Check this guide for a related HTML issue.
10. Section lacks heading. Consider using “h2”-“h6” elements to add identifying headings to all sections. 1.60%
The <section> element can be used to define sections of a document, like chapters, tabbed content, etc. Consider using a heading element (any of <h2> to <h6>) to present each section.
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