Typically, you can use the @@extension_id special string to reference the absolute path into the bundled resources of an extension, such as an image or a custom font, in a CSS file.
However, this broke with Safari 18.
Consider this section in a popup.css file:
.card-icon {
height: 16px;
width: 20px;
background-image: url(safari-web-extension://__MSG_@@extension_id__/images/card.svg);
background-size: 20px 16px;
}
In Safari 17.4, once loaded in the browser, @@extension_id is replaced with E8BEA491-9B80-45DB-8B20-3E586473BD47, and the background-image reads as so:
background-image: url(safari-web-extension://E8BEA491-9B80-45DB-8B20-3E586473BD47/images/card.svg);
But as of Safari 18, the @@extension_id just collapses to an empty string, and the background-image reads as so:
background-image: url(safari-web-extension:///images/card.svg);
and the svg fails to load with the following error: "Failed to load resource: You do not have permission to access the requested resource."
This is a regression, does to match the behavior of the other major browsers, and should be fixed.
Filed with Feedback ID: FB15104807
Thanks for the bug report! This is a bug, and we plan to fix it as soon as possible.
In the meantime, you can use relative URLs to load the font and image. This will work in both new and old versions of Safari, as the CSS file is loaded as a safari-web-extension URL.
background-image: url(/images/card.svg);
