HTML signatures may be technically possible but are not officially supported.
Everything around HTML signatures is hacky and might lead to unwanted behavior.
Pasting HTML content into signatures is, like everything else with HTML subject to Zammads HTML sanitizer doing things to it. Apart from your href being incomplete (protocol missing). There are several encoding issues on our github repository. You’ll have to wait until these are sorted out I’m afraid.
Ran into this, too, i.e. variables in HTML attributes aren’t not substituted.
Was trying to “emulate” individual signatures and and embed photos through data URI dynamically, but alas…