[im3w1l]

Couldn't resist trying to go further. We can use an implicit viewBox.

  $ echo '<svg xmlns="http://www.w3.org/2000/svg" height="256" width="256"><circle r="1000" fill="#aad3df"/></svg>' | brotli -9 - | wc -c
  83

r="1000" hits brotlis built-in dictionary, but if you target zlib then r="2566" is better.

[dchest]

You can also drop the new line character by using echo -n, which gives 82 bytes :)

[pbhjpbhj]

Can you use a short closing tag "</>" or implicit closure?

[im3w1l]

Those are both sgml (as opposed to xml) things. Neither validates.