On Mac browsers, Command-L should select the address bar
I often use
CMD + H for hiding Safari (or any other application on the Mac). However, while I am editing a post in Stack Overflow I can't use this shortcut because
CMD + H adds a new markdown-header within the editor window. So instead of hiding, it inserts the following
## Heading ##
I personally don't think that a website should cancel out a browser's or the operating system's shortcuts in any case.