Comment by LorenPechtel

Yeah, I think that's the real answer.

Look at what Microsoft did with Excel--the dangerous stuff is behind a switch.

Thus, solution:

Add two bits to the tag.

SVG1 does not execute any sort of script.

SVG2 does not follow links.

SVG3 is actually SVG1 + SVG2 as these are bit flags, not numbers.

Additional bits are reserved for future use if any other issues are found.

The only real safety is in the engine, not by any sanitizer.