I tend to lower-case all my HTML because it has less entropy and therefore can be compressed more effectively. But in case of modern compression algorithms, so

I tend to lower-case all my HTML because it has less entropy and therefore can be compressed more effectively. But in case of modern compression algorithms, some of them come with a pre-defined dictionary for websites. These usually contain the common stuff like <!DOCTYPE html> in its most used form. So doing it like everybody else might even make the compression even more effective.