Pretty much all the major browsers implemented websql, which then never made it into a standard as there were no competing implementations [0]. Nice while it lasted.
JavaScript is a good example. It wasn't the only web scripting language available, there was JScript and Java Applets and VBScript. JavaScript won and now it's ubiquitous.
And for an example of co-existence of competing languages, see javascript triggering flash/actionscript (e.g. for clipboard access) and vice-versa (e.g. for DOM access).
Both are somewhat dated examples, that is pre-HTML5.
Web standards have always been a mishmash of competing standards. Eventually one spec will win out and that will be the next standard.