HTML5 is javascript , and it gets interpreted (converted) into native code ( I think , correct me if I'm wrong ) in the browser , WHILE running the application , that's called an JIT compiler (Just In Time) , and that's why it's slower than native C , aka NodeWebkit ...
I hope this shed a bit of light on Javascript manners
Moreso , if you want to go more into details ,
Almost correct, interpreting it would mean running the JS without compiling it into native code this is the more traditional slower "web" method of running JS.
The V8 engine (Chrome) actually compiles the JS to native machine code X86-64, ARM etc which has a number of advantages eg it can be further optimised at runtime, be cached etc etc It's almost as fast as native because it is ultimately native there's a loss of time in the initial conversion one must assume. Thing is although its been out in the wild a few years now the V8 engine is imo very much a work in progress I expect to see further significant improvements in time.