You can make games on a web page with 2D (<canvas>, SVG) or 3D (WebGL) graphics, 3D sounds, controller input, etc.
It will usually not perform as well as something that runs directly on the desktop, but this might not matter to you (this is usually the case!)
One of the nice things is that you can alleviate the DOM (HTML) to make GUI, which is a bit annoying to make manually with code.
The main drawback is that web technologies are advancing fast, which results in different browsers not necessarily supporting everything, browser-specific prefixes, etc. (plus v8 vs SpiderMonkey on the JavaScript side - or if this means nothing to you, why Chrome is so much faster than Firefox at executing JavaScript).
You can check
caniuse to see what's supported by what browser, and
webglstats for WebGL specific stats.