Inevitably, they will continue to present features that are browser specific. Some of those will be accommodated in the next revision of the ECMA standard. But some won't.
You may find that your script needs to do something more esoteric than a simple form validation or mouse rollover. In particular doing any dynamic HTML and changing <DIV>, <SPAN>, or <LAYER> content. For these kinds of things, you may need to use browser-detection scripts and then provide alternative versions of some of your functions so that you select one that is appropriate for the browser the script happens to find itself running in.