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How to use a context

  1. Identify context.
  2. Determine whether to fetch resource based on CSP directives and type hint, if any.
  3. Fetch resource.
  4. Set no-sniff flag on resource (based on URL), if necessary.
  5. Handle resource.
  6. Sniff resource.
  7. Process and display resource or prompt to download resource, as appropriate.

Context types

Context Definition Used in HTML? Used in CSS? Scriptable? CSP Directive Type Hint Sniffing Algorithm
browsing (navigate) HTML Yes No? Yes MIME type sniffing algorithm
nested browsing (navigate) HTML Yes No? Yes frame-src
connection Yes No? Yes? connect-src
image Yes Yes No img-src rules for sniffing images specifically
audio/video Yes No? No media-src rules for sniffing audio and video specifically
plugin Yes No? Yes object-src rules for sniffing in a plugin context
style Yes Yes? No style-src @type or "text/css" rules for sniffing in a style context
script Yes No? Yes? script-src @type or "text/javascript" rules for sniffing in a script context
font No Yes No font-src format() rules for sniffing fonts specifically
text track Yes No No "text/vtt"
cache manifest Yes No No "text/cache-manifest"

Context triggers

Context HTML Triggers CSS Triggers Other Triggers
nested browsing <iframe>, <object> (sometimes), <frame>
connection EventSource, WebSocket, XMLHttpRequest
image <img>, <link rel=icon>, <input type=image>, <object> (sometimes) background-image
audio/video <audio>, <video>
plugin <embed>, <object> (sometimes)
style <link rel=stylesheet> @import?
script <script src>
font @font-face FontFace#load(), FontFaceSet#load()
text track <track>
cache manifest <html manifest>


  • What about SVG?
  • What about XSLT?
  • document.load()? (Seems similar enough to connect-src.)
  • What about CSS masks?
  • What about CSS shapes?

See also