The definition of "inline" elements in §6.1.4 and §7.4.3 is a little fuzzy.

For XHTML, the definition is

The inline elements are those elements recognized as HTML elements that are in the %inline category of any of the XHTML 1.0 DTDs, in the %inline.class category of the XHTML 1.1 DTD, those elements defined to be phrasing elements in HTML5 and elements recognized as HTML elements with local names ins and del if they are used as inline elements (i.e., if they do not contain element children).

The use of a list of sets joined with "and" suggests that there is a single set of inline element (names) which is the union of these sets, and that the set of inline elements is therefore independent of the HTML version, except to the extent that the rules for an element being "recognized as an HTML element" are dependent on HTML version.

For HTML, the definition is

The inline elements are those included in the %inline category of any of the HTML 4.01 DTDs or those elements defined to be phrasing elements in HTML5, as well as the ins and del elements if they are used as inline elements (i.e., if they do not contain element children).

The use of "or" here suggests that we use one set of names or the other, presumably the first set if it's HTML4 and the second set if it's HTML5.

There's no mention here of the element being "recognized as an HTML element".

In practical terms, suppose we have a <mark> element and we're serializing as HTML 4.0. The <mark> element is new in HTML5. Do we treat it as an inline element or not, and is the answer the same for HTML and for XHTML?

I think the solution that will work best for users is to have a single set of element names considered to be inline elements, which is the union of the above categories, and which is used both for HTML and for XHTML, irrespective of version.