A colleague of mine "wrote" a H265 decoder in Javascript.

I put that in quotes, because it was a byproduct of a much cleverer project: http://www.argondesign.com/products/argon-streams-hevc/ ; he wrote a parser for the "human readable" H265 specification, using the O-Meta system. This supported a number of pluggable backends, one of which emitted a javascript program, and one of which was the actual saleable product of an H265 decoder validation test suite.