Create an HTML email signature in TextEdit or some other rich text editor on macOS, copy it and paste it on iOS using Continuity Clipboard into the signature text field in iOS’ Settings -> Apps -> Mail -> Signature text field, and pray that it works.
When it does work, it ignores text size. Want your email signature to be in a smaller font size? Too bad.
The only workaround I’ve found is to copy an email signature from another email account that miraculously did copy over successfully, copy it on iOS and paste that in to another’s email signature field, and manually rewrite the whole thing.
Of course, the HTML links aren’t editable, and the Signature text fields don’t enable the “Add Link” item to the hover tools menu when selecting text.
Worst of all, even when the email signature is finally perfect, the Signature text field doesn’t preview it properly. If text is pasted in at a smaller text size, the preview still shows it at the default size. The only way to accurately preview an email signature is to draft a new email and hope the email signature displays like it did originally in TextEdit.
In short, never create a new HTML email signature on iOS. Create in on a Mac (source), copy it over the network with Continuity Clipboard, hope it works, and treat the iOS email signature fields (destination) as a never- or rarely-editable workspace.