Each attribute belongs to a category. And for some types, their values are restricted. Here are the categories of attributes:
To find the type of the attribute, search the tag name (here) and check its definition.
<a> attribute "id" has invalid value "567"
The attribute ID is of type ID. As described above, it should begin with a letter
BAD <a id="567" href="/index.html">
GOOD <a id="n567" href="/index.html">
W3C spec: http://www.w3.org/TR/html4/types.html#h-6.2
Tags definitions: http://www.w3.org/TR/html4/index/elements.html