In XHTML strict mode, which basically nobody used.
In XHTML transitional, and HTML1-4, what you'd get is browsers with divergent understandings of your DOM tree structure such that content that worked fine in one would be a horribly mangled mess (often breaking layout in difficult to read ways) in another browser.
Some of us tried XHTML. A little bit of extra consistency in syntax and closed tags was rewarded with being able to use tooling built around XML expectations. I dug that.
And then I found out there were some browsers that wouldn't handle it right unless content-type headers were set up right on the server, and of course that made it an extra pain, especially on commodity hosting where you might not have that remotely under your control.
The "strict warnings"/"rendering problems crash the page" setting was also... mixed. Certainly prompted you to pinpoint the issue and fix it but that level of nag often felt unnecessarily militant after what everyone was used to.
In XHTML transitional, and HTML1-4, what you'd get is browsers with divergent understandings of your DOM tree structure such that content that worked fine in one would be a horribly mangled mess (often breaking layout in difficult to read ways) in another browser.