Value of attribute ... invalid: ... cannot start a name

Cause:

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

Example:

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

Good<a id="567" href="/index.html">
Good<a id="_hello" href="/index.html">
Good<a id="n567" href="/index.html">

Solution:

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

References: