While it is mostly marketing (i.e. still using Chakra and Trident engines), the biggest change from a web developer's perspective is that the new Spartan browser will be evergreen[1].
EdgeHTML.dll is just a fork of mshtml.dll, which started somewhere after IE11 and stripped out all the compatibility modes (except quirks and limited-quirks, both of which are starting to get properly standardised) and then started refactoring the code to get rid of all the less nice design decisions caused by having to keep the IE7 code working through a bunch of ifs.
IE has been evergreen since version 10. The biggest change is that this is not Trident. It is a new rendering engine, called EdgeHTML, which started as a fork of Trident. That might sound odd, but Trident and the rest of IE had accumulated years of technical and backwards-compatibility debt: All the different document modes and obscure features from the ActiveX era that couldn't be removed because enterprise intranet sites relied on them.
So what Microsoft have done is create an entirely new browser, using the EdgeHTML fork, while keeping IE around only for enterprise installs. Then they went on a killing spree in the EdgeHTML source, ripping out all the crap that they were previously stuck with, then adding new features into their newly cleaned-up codebase. This will then form the basis for the new, consumer facing browser in Windows 10.
They should have done this EdgeHTML rewrite/legacy cruft split with IE9 rather than layering on more kludges.
They should eliminated native code apps in user mode in Vista and forced developers to ship only managed code with legacy native code apps running in a virtual machine sandbox.
They should have finished WinFS and transcended the files and folders thang. (No, SQL Server does not count as WinFS, it should have been THE file system in Vista.) They should have fixed the file locking thing on open files, geez I hate hate hate that DOS backwards compatibility.
They should have done something/ANYTHING amazing with WinCE beyond pocket office with the insane lead they had. I was running .exe's and playing DOOM at a decent clip on my Samsung phone in, when, 2004?
There's so many wouldda couldda shoulddas I have lined up in my mind wrt Microsoft. Makes me sad.
I truly hope Microsoft wants to win this new browser war. I truly hope there is some fire left in you M$. Show me what money combined with hardcore computer science can really achieve!
[1]: https://plus.google.com/+PaulIrish/posts/f15yUhu4tE3