On A List Apart today is an article on how IE8 will selectively enable its new rendering engine. The executive summary is that pages will now be asked to declare what version of each browser they were developed against, and a browser will then -- in theory at least -- select the most appropriate rendering engine.
I'm sure many people will look at this and have reservations. However, I for one am pleased to see this. All of this screwing around with DOCTYPE sniffing in previous versions left a bad taste in my mouth, because it was adding additional meaning to something where it did not exist before and in a way which differed from browser to browser. They thought they were asking "what version of HTML does this document target?" but that was the wrong question. The question was "what set of rendering engine bugs does this document target?".
Rendering engine bugs are by definition specific to a given rendering engine. There can be no standard way to select for these, because they are by definition non-standard. It is for this reason that I'm glad to see this issue being tackled in an honest fashion: document authors are now asked to list the set of rendering engine bugs they've tested their documents against, with a separate version number for each rendering engine.
Can't really get much clearer than that.