I've found that high battery drain is usually caused by applications running in the background.

In particular, I've found micro-b browsers open on certain websites cause a big drain, while other websites don't. Sometimes this is caused by animations, but I have also seen examples where that didn't seem to be the case.

I am guessing that another cause is faulty/badly written javascript, that doesn't block as much/often as it should (I've also seen this using firefox on the desktop).

So beware web pages! Only keep open ones you trust!