It's browser dependant.
Internet Explorer 10 and earlier does NOT support WebGL.
But IE 11 and onwards DOES.
Chrome, Firefox, Safari and Opera (the other major PC browsers) all supports WebGL.
It's NOT dependant on operating system as winsonzhong indicates, but solely on the browser installed.
Construct 2 does, however, have Canvas fallback, which is supported from earlier versions of Internet Explorer such as 9 and 10.
Personally, I'm doing an Internet Explorer version check; if earlier than 11, the visitor will get a "please choose another browser" screen, with links to Chrome, Firefox and Opera.
Worth noticing; WebGL version 2 is coming soon - and it'll have a lot more features and better performance than the current WebGL version 1.