id="x" (valid)
id="#x" (valid)
id="%23x" (valid)
href="#x" (valid; jumps to id="x" in Chrome, Mozilla and Opera, but id="#x" in IE)
href="##x" (invalid; jumps to id="#x" in all Browsers
href="#%23x" (valid; jumps to id="#x" in Mozilla and Opera, but id="%23x" in Chrome and IE)
href="#%2523x" (valid; jumps to id="%23x" in Chrome, Mozilla and Opera, but nowhere in IE)