More than one page. Sounds trivial, but having more than one web page also means you'll have navigation, which is much more challenging on default browser styles than simple document markup.
It's almost a shame the idea of having a sitemap.xml which the browser could parse and provide native navigation for never took off.
Of course it didn't because native controls are the first thing any designer changes because native controls on the web were always associated with amateur websites.
To be fair native controls were total ass for years. And still kind of are. Doubly so if you're trying to build an "app". Basic crap any half-decent UI toolkit would provide out of the box, just not there at all. The stuff that is there's often not great.
It seems like as soon as it became common to work around problems with HTML ui in Javascript, the already-slow work to improve the standard appearance and functionality of HTML websites dropped to almost zero. We got some alright semantic-markup stuff after that but it took forever to even get a couple layout primitives that are non-terrible.
