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In-Browser Performance Linting With Feature Policies

Here’s a neat idea from Tim Kadlec. He uses the Modheader extension to toggle custom headers in his browser. It also lets him see when images are too big and need to be optimized in some way. This is a great way to catch issues like this in a local environment because browsers will throw an error and won’t display them at all!

As Tim mentions, the trick is with the Feature Policy header with the oversized-images policy, and he toggles it on like this:

Feature-Policy: oversized-images ‘none’;

Tim writes:

By default, if you provide the browser an image in a format it supports, it will display it. It even helpful scales those images so they look great, even if you’ve provided a massive file. Because of this, it’s not immediately obvious when you’ve provided an image that is larger than the site needs.

The oversized-images policy tells the browser not to allow any images that are more than some predefined factor of their container size. The recommended default threshold is 2x, but you are able to override that if you would like.

I love this idea of using the browser to do linting work for us! I wonder what other ways we could use the browser to place guard rails around our work to prevent future mistakes…

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