We do have some information about format support from Accept & User-Agent headers in regular image requests. Chrome for example sends `accept: image/webp,image/*,*/*;q=0.8`, indicating that it prefers WEBP where available, followed by other image formats. Firefox sends `accept: */*`, which at least indicates that it does not yet support webp. 

However, there is no information about SVG support levels in common browser's accept headers. While [basic support is rather good across the board at 97+%](http://caniuse.com/#search=svg), support for advanced features is less common and mature. We can't communicate very detailed information about features used in a given SVG, so I agree with you that the conservative option will be to default to bitmap thumbs when a size is requested, and let clients opt in to using the SVG original when they are sure that client support is decent. Some indication of SVG original size & render effort could be helpful for this decision, but we'd need to add more metadata to make this available client-side.

Let me sketch how this could work with a combination of negotiation & explicit parameters:

- Supply original format and size in the URL or metadata, and let the client choose between SVG originals & bitmap thumbs. Example using URL syntax: `/someimage?o=vec`
  - `o=vec` or `o=vid` (metadata): Original is a vector image or video. Can consider omitting this for bitmaps (default).
  - `oh=1024&ow=768` (metadata): Original width & height. Absent for SVGs when API or HTML use case recommends using SVG original.
  - `w=640` (server / client controlled): Request a bitmap thumb with this width. Thumb URLs served by APIs or HTML content can recommend a bitmap or original SVG format by adding / omitting the thumb width parameter.
- Use basic content negotiation based on accept and user-agent headers at the Varnish layer to select the most appropriate low-level bitmap image format. In particular:
  - Serve WEBP only when the client explicitly indicates support in its `accept` header, and the UA does not match a blacklist of browsers with incomplete WEBP support. PNG and JPG are used as the safe baseline.
  - Can also take into account quality parameter.