Hello Kristin,
as HTML5 is a web standard, but not a filetype, this is a bit difficult to answer.
You could save a file with the ending .html5, but this would be a classic text file which could include HTML, CSS & JavaScript code. This would only be visible as expected when opening in a browser. Besides that, it could be possible, when viewing, that external code and / or assets like images is triggered and loaded.
On that basis, something like this is not suitable as a classical proof in the sense of Workfront.
So, I assume that, in the moment your designers save to PDF the whole result is rendered first and saved as PDF.
I hope this helps.
Regards
Lars
