> We do use the relevant parts of HTML and CSS, where appropriate. But web standards do not provide for specification of the layout of the document in a robust way, which is guaranteed not to reflow when opened on other systems. Furthermore, browser technologies are a moving target, with implementations changing very rapidly. Therefore, they do not provide a suitable basis for archival documents.
https://editablepdf.org/faq/
> Why not use web standards, such as HTML/CSS?
> We do use the relevant parts of HTML and CSS, where appropriate. But web standards do not provide for specification of the layout of the document in a robust way, which is guaranteed not to reflow when opened on other systems. Furthermore, browser technologies are a moving target, with implementations changing very rapidly. Therefore, they do not provide a suitable basis for archival documents.