It proves that engineering is hard and that everything has costs and benefits, and you can't make them go away by just ignoring them. It turns out that "being robust" had a lot of costs we didn't see.
It also shows the harmfulness of binary black and white thinking in engineering. There are choices other than "just let everyone do whatever and hope all the different things picking up the pieces do it in more or less the same way" and "rigidly specify a spec and blow up the universe at the slightest deviation". Both of those easy-to-specify choices have excessive costs. There is no escape from hard design tasks. XHTML may always have been doomed to fail, but that is not to say that HTML had to be allowed to be as loosey-goosey as it is, either.
Had a gradient of failure been introduced rather than a rigid rock wall, things very likely wouldn't have gotten as badly out of hand as they did. If, for instance, a screwed up table was specified to deliberately render in a very aesthetically unappealing manner, but not crash the entire page the way XHTML did, people would have not come to depend so much on HTML being sloppy. The resulting broken page would still be somewhat usable, but there would have been motivation to fix it, rather than the world we actually live in where it all just seemed to work.
It also shows the harmfulness of binary black and white thinking in engineering. There are choices other than "just let everyone do whatever and hope all the different things picking up the pieces do it in more or less the same way" and "rigidly specify a spec and blow up the universe at the slightest deviation". Both of those easy-to-specify choices have excessive costs. There is no escape from hard design tasks. XHTML may always have been doomed to fail, but that is not to say that HTML had to be allowed to be as loosey-goosey as it is, either.
Had a gradient of failure been introduced rather than a rigid rock wall, things very likely wouldn't have gotten as badly out of hand as they did. If, for instance, a screwed up table was specified to deliberately render in a very aesthetically unappealing manner, but not crash the entire page the way XHTML did, people would have not come to depend so much on HTML being sloppy. The resulting broken page would still be somewhat usable, but there would have been motivation to fix it, rather than the world we actually live in where it all just seemed to work.