How I Fixed Flash of Unstyled Content (FOUC) in Shopify Collection Filters (Step-by-Step Debugging)

Flash of Unstyled Content (FOUC) is one of those frontend issues that looks small but can completely break the user experience. Recently I had to fix this problem in a Shopify store where the collection filter panel briefly appeared as raw HTML before the page finished loading. The store was running a paid Shopify theme with several custom edits and third-party apps, so the source of the problem was not obvious. In this post I’ll show how I tracked down the cause and fixed it without breaking the theme. The problem Every time a user opened a collection page, the filter sidebar appeared for a split second without styling. After that, the CSS loaded and everything looked normal. This happened on: Desktop Mobile Slow connections First page load This is a classic FOUC (Flash of Unstyled Content). Possible causes In Shopify themes, FOUC usually happens because of: CSS loaded too late JavaScript modifying DOM before styles load App scripts injected in wrong order Deferred or async assets Dup