It can be done! There are some gotchas, however. The page htxt links to is fine, as far as it goes, but please note the following:
1) Both script and canvas elements must have id attributes. The init function uses these attribute id's to associate a given script with a given canvas. I found the simplified init function easier to understand than the official one. You will want to master the official one if you have multiple canvases on one page.
2) If you use internet-style color designations, like #23ff9a, watch out! IE 8 wants all upper case hexadecimal color numbers from Processing.js/canvas. Write #23FF9A! This is what the documentation shows, so it shouldn't be a complete surprise. The error is a sometime thing, which makes it crazy to figure out. Mostly, larger numbers (for lighter colors) with lots of f's seem to be afflicted. White, #ffffff, is OK, but #ff00ff is not. Firefox and Safari are case-insensitive in this regard. The documentation says you can use an alternate hex notation with alpha channel (the CC) that looks like 0xCC006699. This didn't work for me; maybe it's on the to-do list.
3) The .equals() method on strings is missing! Andor Salga, one of the Seneca College crew working on Processing.js, wrote a simple boolean stringsEqual(str1, str2) function you can see here. This will do until the matter is definitively fixed.
4) It's not true that stroke() doesn't work with excanvas.js. It does. However, if your Processing.js code has even one little syntax error (I can't really categorize which kinds, but trying to use .equals() will do it) your routine will probably fail silently in IE8, whereas, in Safari or Firefox, your rectangles may lose their outlines, i.e. stroke() will quit working. IE on Vista, and Safari on the Mac, have both exhibited stronger syntax checking than Safari or Firefox on Vista, which will blow by certain errors and render a defective graphic.
5) Text, invoked using the text() function, renders in Firefox (in an unchangeable font of Firefox's choosing), but, as far as I can tell, not in IE8 or Safari. The Glyph Method is suggested here. The code is in place, but getting the fonts looks like a problem. Inkscape looks pretty impenetrable to me. As far as I can tell, what is needed is a lot like old pen-plotter fonts - a vector path with pen-up and pen-down commands between runs of nodes. Turns out FSF/GNU has some that might be massaged into the right format without too much trouble. I don't know where the format is defined, but it's probably over at W3C somewhere. The approach with real potential for presentable fonts is the IE/VML wing of Cufon. See How does it work? I really want this last link in the chain, but I could use some help.
Processing.js is one whale of a project that deserves our support. It has enormous potential. I would encourage you to pitch in if you are able.
