This PR fixes two bugs causing HTML validators to complain.
- Error: Bad value data:image/svg+xml;charset=utf-8, for attribute src on element img: Illegal character in scheme data: < is not allowed.
- Fixed by using base64 for svg during `layout=intrinsic` to avoid angle brackets
- Error: Element img is missing required attribute src.
- Fixed by using base64 transparent gif for `loading=lazy` placeholder
Fixes#18850
