Scale down to fit an image in FOP

I found this page here explaining how scaling works with XSL-FO.

Essentially, this is the snippet that I used to make it work:

<fo:external-graphic
    src="https://stackoverflow.com/questions/6220316/url("https://stackoverflow.com/questions/6220316/...")"
    width="100%"
    content-height="100%"
    content-width="scale-to-fit"
    scaling="uniform"
    xsl:use-attribute-sets="img"/>

I found that width="100%" was the missing piece in your own attempts. Hope this helps future visitors.