You don't need the quotes in many cases.
Instead of:
<link rel="icon" href="data:,">
Try
<link rel=icon href="data:,">
Instead of:
<link rel="icon" href="data:,">
Try
<link rel=icon href="data:,">
Another fun trick is using <!doctypehtml> since the spec says to pretend a space is there if not present for whatever reason (https://html.spec.whatwg.org/multipage/parsing.html#parse-er...)
The first error read "Bad value data: for attribute href on element link: Premature end of URI."
The 2nd error read "Missing space before doctype name."
Depending on the context these hacks may still be useful, but I personally think that both production sites and code golfing should require valid HTML.