Maybe not only PDF is up to the task, but this site doesn't show that web would be. In section "The quantum circuit model", look how
1/sqrt(2) looks (the 2 falls out of the sqrt sign). Then there is inline maths using the font "Noto Serif", which isn't suitable for writing formulas and makes it look really inconsistent when taken with the formulas which stand in their own lines.
ℂ uses "Noto Serif" even in dedicated lines - the straight line with the rounded line couldn't be closer together. This font just isn't up to the task. It makes the formulas both less legible and ugly. Inconsistent spacing in "|0>" looks really off. A lot of inline formulas look as if they were about to fall off their lines. I don't get the use of sans-serif for section titles - default serif fonts in LaTeX make section titles both stand out and look like the belong in their surroundings --- this one doesn't have that effect. Hyphenation algorithm is also suboptimal. Look at the original PDF - at the same time there are fewer occurrences of hyphenation and the space between words in paragraphs is more evenly distributed. Also, the hyphens are easier to spot.
What PDF-s generated by LaTeX have, that so far web failed to replicate, is beautiful fonts for maths, consistency, legibility and overall beauty of the end result. They also have the advantage that they are self-contained, so if I want to archive one or move to another place, I just grab the single file and I'm done. With web a single document is distributed across many files and you need make a weird wget dance (which is easy to misuse)/install some obscure browser extensions/etc - not exactly user-friendly. Also, if you want to print the document to get away from the distraction of the computer, you're guaranteed to get at least as good a result as what you see on the screen.
Don't get me wrong. This attempt is better than any that I've seen so far. Kudos to the author for that. It's no small feat. But it doesn't prove that PDF-s are obsolete and that people favouring them are irrational.