Cause:

Each attribute belongs to a category. And for some types, their values are restricted. Here are the categories of attributes:

• CDATA is a sequence of characters
• ID and NAME must begin with a letter ([A-Za-z]) and may be followed by any number of letters, digits ([0-9]), hyphens ("-"), underscores ("_"), colons (":"), and periods (".").
• IDREF and IDREFS are references to ID tokens defined by other attributes. IDREF is a single token and IDREFS is a space-separated list of tokens.
• NUMBER tokens must contain at least one digit ([0-9])

Example:

<a> attribute "id" has invalid value "567" The attribute ID is of type ID. As described above, it should begin with a letter

<a id="567" href="/index.html">

<a id="_hello" href="/index.html">

<a id="n567" href="/index.html">

Solution:

To find the type of the attribute, search the tag name (here) and check its definition.

References:

• W3C spec: http://www.w3.org/TR/html4/types.html...
• Tags definitions: http://www.w3.org/TR/html4/index/elem...