I like to think of HTML as a design constraint. We can use CSS to enforce that constraint by writing selectors that leverage the semantics of our documents.
The most compelling point of Heydon’s work for me is the reliability of using such selectors. If we apply
class="btn" to a
<button>, but somebody later changes the element to a
<span>, we have lost the meaning of the element with no visual clue of the side effects.