Beautiful, and would be so much more beautiful if it used a modern font, without these weird cs and fs. I actually had to copy-paste "reproduction" to convince myself that this is a c character, and I'm not reading something in some ancient version of english with characters that I've never seen.

The Old English representation, inclusive of the long S, is consistent with the 1847 source material, as you can see here: https://archive.org/details/firstsixbooksofe00byrn/page/n21/...

The first printed copy, of 1482, was in Latin: https://www.loc.gov/item/2021667076/

The Greek (Ελληνικά) representation is presumably consistent with Euclid's original manuscript, e.g. as represented in this copy, of 888: https://historyofinformation.com/detail.php?id=208

There is a menu from the top bar to change the "Modern English."

You get used to it very quickly. If you're actually interested, give it five minutes and you'll read it like normal.

If you're not interested, the long s and the ligatures are not the problem.

> in some ancient version of english with characters that I've never seen

There are really only 3, or if you want to stretch it 5, English characters that you might not be familiar with as characters: þ, ƿ, ð, æ, and ȝ. And the last of those isn't even present in Old English; it's a Middle English thing.

If you were reading something in Old English, the use of ƿ wouldn't really be an issue - the issue would be that nothing made any sense. Recognizing the characters used, or not, is irrelevant.

As you note, the recognition issue is an issue of fonts and not of alphabets. Compare wikipedia's sample of blackletter: https://commons.wikimedia.org/wiki/File:Calligraphy.malmesbu...