HTML5 while on the surface doesn't look or seem to act any differently to HTML4 (or earlier)., is the entire reason that no one uses Flash or Silverlight anymore.
And keep in mind., you're just scratching the surface here.
I mean depending on how deep you want to delve down the rabbit hole (say with Core.js and Angular.js... both are Free Libraries; which are akin to the Win32 API or such for HTML5)
well there is A LOT that you can do to create a highly interactive experience.
Like do you remember years ago when the Forum was gripped with Legend of the Green Dragon? (A PHP Multi-User Dungeon / MMORPG)
Well that worked heavily on being Server-Side, with the Pages having various Templates that were then created and dispatched to the "End Users" (i.e. Players)
But with HTML5., you can essentially create that as a Single MVC Page Solution; with Graphics that Animated based on what's going on... Direct Peer-to-Peer PVP... Real-Time Chat, Events, Actions, Interactions between Players, etc.
All a Server would need to handle today would be keeping storing the Asset Resources and Player Data.
I'm sure that W3Schools (which was a great HTML/JS/CSS/etc. learning site in the past) is still a great place to start.
And as noted there are some good (Free and Open) Libraries that are fairly plug-and-play, so that you can rapidly create something usable without having to manually handle all of the backend.