This CSS feature is now available in *most* browsers that we support, with old Chromium-based browsers being the only exception; please see https://developer.mozilla.org/en-US/docs/Web/CSS/color_value/color-mix#browser_compatibility
From this data we see that the feature in question has been supported since Chrome 111, which was released on 2023-03-01 (i.e. almost two years ago).
Please note that we've never guaranteed that all features and functionality will be available in the oldest supported browsers.
Furthermore, even with the `color-mix` fallback removed PopupAnnotations will still function just as before but may render with the default color (defined in the CSS-file) rather than the one specified in the PDF document.
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