A core attribute is an attribute that is allowed in all tags. In the current release of b2evolution, "class" is defined as a core attribute, but there should be more.

Have you ever tried to add inline CSS to your blog entries, by adding a "style" attribute? If so, then it's likely that the HTML-checker has spit out an error message, saying something to the effect:

Please correct these errors:

Tag [your tag] may not have attribute style

Likewise, I've discovered similar problems using "title" and "id".

After manually adding "style" as an attribute for many individual tags, I finally clued into the fact that the coreattr feature (around line 103 in the _formatting.php file) would do all of them in one fell swoop.

It's right below the comment that reads

// Allowed Attribute Classes

I've changed my core attributes line (and you might like to as well):

// Allowed Attribute classes
define('A_coreattrs', 'class title style id');

I hope this helps.

-stk :D