... attribute "..." has invalid value "..."

Cause:

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

  1. CDATA is a sequence of characters
  2. 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 (".").
  3. 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.
  4. NUMBER tokens must contain at least one digit ([0-9])

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

Samples:

<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">

References:

W3C spec: http://www.w3.org/TR/html4/types.html#h-6.2
Tags definitions: http://www.w3.org/TR/html4/index/elements.html