It's not a huge benefit because they are simpler; they are usually
equivalent to the old standard. The benefit comes from newer standards
being a lot more precise.
e.g. HTML4 did not specify what to do with invalid markup, which makes
writing a conformant parser easier. In practice, many websites weren't
valid HTML4, so you had to reverse engineer whatever the other parsers
did with invalid markup.
HTML5 doesn't really have a formal grammar, it's specified as an
imperative tokenizer and parser. It actually takes somewhat longer to
implement than HTML4, but it doesn't suffer from compatibility issues.
OTOH there are new problems with the "modern" standards that old ones
did not have:
* It's unversioned, updated pretty much daily; insert walking on water
quote[0].
Random example: I added support for the (ancient) document.write API
to my browser a few months ago. Recently I looked at the standard
again, and it turns out my implementation is outdated, because there's
a new type of object in the standard that must be special cased. Many
similar cases; this particular one I don't mind, but it shows how hard
it is to just stay fully compliant.
* It's gigantic and bloated, full of things nobody ever uses. If
something gets into the standard, it typically won't ever get removed,
and WHATWG has operated under this policy for more than a decade. So
implementing it from start to end takes way too long, and the best
strategy to get something useful is to "just" implement features that
websites you use will need.
* Above is the WHATWG model, which applies for HTML and DOM, but not
CSS. The W3C model (used in CSS) has versioning, but they broke it in
a different way: there is no comprehensive "CSS 3 standard", just a
bunch of modules haphazardly building on top of each other and
CSS 2. Plus it's much less precise than the HTML standard, with basic
parts left entirely unspecified for decades. See the table module[1],
or things like this[2].
[0]: "Walking on water and developing software from a specification are
easy if both are frozen." - Edward V Berard
[1]: https://drafts.csswg.org/css-tables-3/ still in "not ready for
implementation" limbo after years.
[2]: https://github.com/w3c/csswg-drafts/issues/2452 - "resolved" by an
unclear IRC log(?), but never specified to my knowledge. It's not
an irrelevant edge case either, Wikipedia breaks if you get it wrong.