Indeed -- in builds where we support `-webkit-filter` (i.e. after the regression range), the site adds `css-filter-enabled` to the `<body class="...">` element. If you remove that class in devtools, then the blue effect "starts working" (presumably using an overlay or something instead of a CSS filter). So this is, in fact, not a regression - just the site using two different versions of the same effect, one with a filter and one without. And the one with a filter is apparently not working due to bug 376027.
Bug 1596979 Comment 8 Edit History
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Indeed -- in builds where we support `-webkit-filter` (i.e. after the regression range), the site adds `css-filter-enabled` to the `<body class="...">` element. If you remove that class in devtools, then the blue effect "starts working" (presumably using an overlay or something instead of a CSS filter). So this is, in fact, not a regression - just the site using two different versions of the same effect, one with a filter and one without (and choosing between them by feature-detecting support for `-webkit-filter`). And the version with a filter is apparently not working due to bug 376027.