The break meant bringing up a new Web rendering engine, free from 20 years of Internet Explorer legacy, which has real-world interoperability with other modern browsers as its primary focus – and thus our rallying cry for Windows 10 became “the Web just works.” This pragmatic viewpoint, which was initially proven out by our work in Windows Phone 8.1 Update, meant that Web standards would continue to be important but should function in the background to drive real-world interoperability between browsers. - A break from the past: the birth of Microsoft’s new web rendering engine, blogs.msdn.com
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I’m always rooting for IE to get better as Internet Exploder edge cases define my job. That said, using Windows Phone 8.1 as your yardstick? 

  • Windows Phone 8 and 8.1 both have some strange behavior regarding hover states. MS tries to spin this as building websites correctly, when Android and iOS browsers have no issues. Correct or not, MS doesn’t have the marketshare to define “correct” anymore.
  • Windows Mobile 8.1 has some fixed positioning issues that Windows Mobile 8 did not. 
  • Windows Mobile doesn’t have Inspect Element. We’re in 2015. iOS has had it for years, Mobile chrome has had it two or three years. IE requires javascript injection for 3rd-party, less functional, inspect element tools.
  • Windows Mobile 8.1 requires EBT to simulate. iOS, Android, Windows (desktop) do not. This means the few Core 2/Core Quad windows boxes in the office cannot simulate it, and must be run inside one of our MacBook Pros as a Simulator within a VM, meaning double overhead.
  • Getting the simulator requires downloading the entire 15 GB VS Express library. Xcode requires roughly 4GB all said and done to run the iOS simulator. Android with GenyMotion is roughly 2-3GB. Both allow for inspect element. 
  • Native devices can be connected for development: Plug in your iPhone or Android phone into your Mac and you’re ready to do front end development in real time. Windows Mobile? You can plug away, but VS Express won’t assist you for front end web development.

These are just my complaints I have at the tip of my tongue. IE11 for the desktop certainly a welcomed change and the IE team seems dedicated to forward motion. Modern.ie is a great start but If MS wants the Web to just work, they need to throw the bone to the front end developers / ux developers to make it possible for them to develop a web that just works.