Post 2 in a running series of questions for DEV!

How do you tailor your native/web apps for quirks of each OS/browser they might run on? For native apps/games, shortcut conventions are different enough between OSes, so if you want to use the standards, you have to first figure out what OS you're on. (Notice that I didn't say "keyboard shortcuts", because of things like long-press on mobile OSes.) Likewise, browsers support a different set of web features differently, for which feature detection is usually used (in JS, anyway). What adaptations and/or techniques have I not thought of? What do YOU use personally?