Top 10
HTML Issues

Web developers worldwide have found
277 million HTML issues
on 9 million checked web pages.
Here are the most common issues detected by Rocket Validator.

1. Trailing slash on void elements has no effect and interacts badly with unquoted attribute values. 50.87%

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 “type” attribute is unnecessary for JavaScript resources. 11.38%

The default type for <script> tags is JavaScript, so you don’t need to include the type for JS resources.

3. Malformed byte sequence. 11.08%

The document could not be properly parsed due to malformed characters. Check the document encoding.

4. The first occurrence of ID “X” was here. 6.33%

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.

5. Duplicate ID “X”. 6.31%

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.

6. No space between attributes. 4.84%

Attributes in HTML elements need to be separated by space.

7. Forbidden code point X. 2.73%

A character has been found in the document that is not allowed in the charset encoding being used.

8. Attribute “X” not allowed on element “Y” at this point. 2.53%

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.

9. Element “X” not allowed as child of element “Y” in this context. 2.39%

The element X is not allowed as a child element of Y. For example, a <ul> element cannot have a <div> child element.

10. The document is not mappable to XML 1.0 due to two consecutive hyphens in a comment. 1.54%

Malformed comment syntax has been found, check out the affected comment.

HTML comments are helpful to leave notes about the code right next to it. To create a comment, it must start with <!-- and end with -->.

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