unsized-media policy controls whether documents are allowed to resize
images to their 'natural' size.
When a document contains images or videos without explicit dimensions, the size is
determined by reading the image data. This takes time, and therefore causes the document
layout to change when the image dimensions become known. When a document is disallowed
unsized-media with this policy, its
<svg:image> elements will adopt a
default size (300px X 150px) and will not change size when the image data is loaded,
providing a potentially less juddery user experience during page load.
How to apply this policy
Send the following HTTP header to control the
unsized-media policy, and
disallow it on all origins:
Feature-Policy: unsized-media 'none'
Does it work?
Currently Firefox, and Chromium based browsers, such as Google Chrome, Samsung Internet,
and Opera, are the only user-agents to support Feature Policy. The minimum version that
correctly recognises the
unsized-media policy is: