When optimizing images, the file size should be kept as small as possible. The larger the
download size is, the longer it takes a page to load. Stripping metadata, picking a good
image format, and using image compression, are all common ways to optimize an image's
unoptimized-images is a policy that restricts images to have a
file size (in terms of number of bytes) no more than a specified ratio of the image size
(width * height) on the web page.
When a document is disallowed to use
unoptimized-images policy, its
<img> elements whose file sizes are too big will be rendered as
The default maximum file size of an optimized image is calculated as following:
metadata size limit + byte-per-pixel ratio * image resolution
- For images of one of the modern formats (JPEG, PNG, GIF, WEBP, and SVG)
- The default metadata size limit is tentatively 1KB (1024 bytes).
- The default byte-per-pixel ratio is tentatively 0.5.
- For images of other legacy formats
- The metadata size limit is set to 0KB.
- The byte-per-pixel ratio is set to 0.
How to apply this policy
Send the following HTTP header to control the
unoptimized-images policy, and
disallow it on all origins:
Feature-Policy: unoptimized-images '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
unoptimized-images policy is: