Chrome OS has proven the viability of an HTML5-based OS.
In Mozilla's case it was an inability to define a market more than the technology stack. I had a Flame. While the OS started a little rough around the edges, by Firefox OS 2.5 it was perfectly usable as a daily driver with modest specs by 2017 standards.
Only in the US school system, because it isn't used anywhere else.
Even here, most people buy Chromebooks to replace ChromeOS with an usable version of GNU/Linux, just like we used to do with Windows 95/98 back in the day.
In Mozilla's case it was an inability to define a market more than the technology stack. I had a Flame. While the OS started a little rough around the edges, by Firefox OS 2.5 it was perfectly usable as a daily driver with modest specs by 2017 standards.