Thanks to all the previous posters on this, according to my Google, I have been to this post a few times for help. I just noticed something that might be beneficial to add. In an ad, had a logo that was used multiple times slightly altered, so to save on file weight, I used the <symbol> and <use> parts of svg. It worked in all the browsers when I tested it, but it was flagged invalid when by the Doubleclick Validator.
I eventually figured out that it was because an svg's <use> was referencing a symbol defined in a separate svg on the page. So in one case, I combined the svgs into one (so the svg wasn't referencing something outside of itself) and another case I duplicated the symbol to both svgs (which didn't hurt the k-weight as much as I thought it would).
Hope that helps someone else (or me in the future when I return to this page).