Unfortunately it is impossible to style the contents of an <iframe> unless you have control over the page loaded in the <iframe>, due to cross-domain resource restrictions.

You would need to style the SVG on the page that you are including:

svg {
  max-width: 100%;
  max-height: 100%;

If you don't have access to that page, then you are limited to styling the <iframe> element itself.

Hope this helps! :)

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