It's not PostScript instead of JavaScript. It's PostScript instead of HTML, JavaScript and CSS. That's (IMHO) the more important point. One programming language to write and structure everything instead of a bastardized SGML plus a bastardized Java plus an ad-hoc config add-on.
I'd much rather have (a slightly saner) JS for everything than PostScript plus HTML plus CSS, if I would have to choose.
I'd much rather have (a slightly saner) JS for everything than PostScript plus HTML plus CSS, if I would have to choose.