It’s like making a .txt document with tables and ASCII art and then on my God other text editors use different fonts and the look breaks. Only the most popular, Windows Notepad is supported.
Web was supposed to be bulletproof, easy to archive and implement. If a webpage break because a browser is supporting 99% of super bloated web standards instead of 99.5% of Chrome, there is clearly something wrong.
My rule of thumb is, try to randomly remove some HTML tags and CSS declarations. If whole site break and is unusable because of one/two lines missing, this website is a hack exploiting browser monoculture.