Note: HTML documents may compress better if you use lower case letters for element and attribute names. The reason is that the compression algorithms do a better job for more frequently repeated patterns, and lower case letters are more frequent than upper case ones.

- https://www.w3.org/TR/WD-html40-970708/intro/sgmltut.html

Is this true? Was this true? Is that why a lot of web pages have the tags in lowercase (<em>) instead of in uppercase (<EM>)?