Not just text but fully interactive web-pages that load images and videos.
I'm fairly certain there will always be some sort of compatibility mode, but for a general audience, I'm wondering if in 30 years, browsers (or their equivalent) would have moved to something closer to a canvas rendering model or something that makes accessing this data difficult.
Plus we will be through the 2020s faster than you know it, and with 20 years remaining after that...I mean I can still, today, easily convert & read Microsoft .lit eBooks from the year 2000...so HTML ought to be no problem whatsoever.
(And this providing that one has obtained access to electrical power kept in a post-apocalyptic 200-story underground mega-vault, via keys found in an auxiliary bolthole maybe, but still!)
Seriously speaking, you probably won't even need a browser; it may be faster to go to your favorite app store and grab an HTML reader.
Keep your images and videos linked relatively & stored locally if possible. Ideally the links are not so much href='https://example.com/example-image.jpg' but rather just href='example-image.jpg' for example.
I would also be surprised if IA didn't eventually add a Reader function for rendering arbitrary pasted or uploaded HTML, if there isn't one already (in addition to the Wayback Machine). Their current reader is easily the best web reader I've used for various formats.
