Performance and Integration. Something we all would be happy with but often not in direct association with HTML.
This really boils down to two key elements:
XML HTTP request 2 and web workers - tools built to make web applications yet run like Native applications.
Some of this stuff is very low level technology and I'll link to some good info at that point but the core concepts here around getting data to the screen without refresh (and hacks) and multithreading, just like you are used to with many application languages.
I know that in RPG programs threading is available but seldom used, and for the uninitiated, this is really about letting tasks run without having to wait for them to finish, or slow down the UI as they process. For many people a little more like running a job in batch and just getting a notification once it's complete.
For web apps this means processing can run in the background without having an affect on the system performance as perceived by the user. In short, you can make the app more snappy!
XML HTTP request 2 (what a mouthful) builds on the original version and gets rid of most if its frustrating and hacked limitations.
You no longer have to do all sorts of jacky efforts to deal with cross domain / cross origin requests - it can now be handled in the standard and this is very useful as many of the required hacks were less than secure in terms of the data transmission and what is held within the server.
Form data gets rid of limitations in transfer so it's no longer JSON or URL encoded and is now simple to name value pairs, easier to read and simpler to use, even enabling you to send binary data! Monitoring of uploads using this is also a useful addition to our IBM i web applications.
So while we have been able to hack around some of the issues for years 'HTML5' here offers again a clean simple, well thought out way to solve common issues in app building.
Some details on WebSockets-
And Web Workers -
Especially good article by the Mozilla Foundation guys - https://developer.mozilla.org/en-US/docs/Web/Guide/Performance/Using_web_workers