What does the JavaScript pseudo protocol actually do? [duplicate]

Answer 1

As @ephemient mentions in his comment, javascript in onclick does nothing, it's basically a useless label. The prefix javascript is used in the tag to tell the browser to run the following as javascript, just as if you had entered the same thing into your browser. You can try it in the location bar of your own browser and see, just enter javascript: alert("Hello").

To summarize:

In onclick: the browser expects this to be javascript, so if you enter javascript: as a prefix the browser will say: 'Oh how quaint, you put a label'.

In or in the browser location bar: the browser does not expect this to be javascript, so if you enter javascript: as a prefix the browser will say: 'Oh, I need to run this as javascript'.