Comment by kstrauser

And some of the sloppy parsers rejected valid XML that the strict parsers accepted.

Combined with Firefox and MSIE having different combinations of HTTP headers and doctype declarations to specify XHTML mode, it was way too hard to emit one single doc that was viewable on all common browsers. For a while my life involved regularly chasing down random issues that suddenly made our customers' browsers show an error page instead of the content, even when it was actually valid and passed all validators.

I don't miss this.