The world has been trying to find out how to run the same code on all machines since the first programmer had to port code. Now, we've accidentally hit the answer to all our dreams, ignoring all the work done in the arena like wxWidgets and Qt.
Just thought I'd share, I'm pretty young but I see myself implementing all my normal GUI stuff as websites and think all that time spent working on Qt, wxWidgets, and Managed C++ has gone to waste because nobody will reasonably expect that anymore in a few years. You need user input? Why not output some html and open it in a browser? Only thing you need to do cross-OS is to run (or embed) any browser, then your GUI just works. Browsers also work across OSes, so you can just embed a quick-and-dirty WebKit or Gecko engine and you're set.*
The tech industry that is normally phasing out other industry's knowledge (like how to draw commercial buildings on paper) is phasing out not a language like normal, but a whole section of ALL languages. Some day, resumes will no longer says the author can code a GUI, but instead that he can write HTML/JS/CSS, and the ability to update a browser's GUI will become rare.
* I glossed over the details with embedding browsers, but suffice to say it's been done and there are a million examples online so it's much simpler than creating a cross-OS GUI from scratch.