Debugging Drupal CSS Issues in Aggregated Environments

Staff Reporter

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CSS aggregation issues in Drupal can be difficult to trace, especially when they appear in one environment but not another. In a recent blog post, Mark Conroy outlines a practical debugging workflow for identifying frontend breakages caused by corrupted CSS files, merge conflicts, or syntax errors hidden inside theme code.

Mark explains that when a Drupal site’s frontend breaks only in certain environments, aggregation is often blamed first. However, the real issue is frequently malformed CSS or unresolved Git conflict markers left in files after merges. Because aggregated CSS is combined into a single file, a single error can break everything that follows in the compiled output.

His debugging checklist starts with searching theme files for leftover merge markers such as <<<<<<<. These markers can easily be introduced during team merges and may not be immediately visible during local testing. Once deployed, they can corrupt the aggregated stylesheet and cause rendering failures.

Mark also highlights common syntax mistakes such as missing semicolons or missing closing brackets. While CSS allows the last property in a declaration block to omit a semicolon, missing separators between earlier properties or leaving blocks unclosed can cause cascading failures across unrelated selectors.

The key takeaway is to verify code integrity before disabling aggregation or rewriting styles. In many cases, simple code hygiene checks resolve issues faster than deeper infrastructure debugging.

Reference: Drupal CSS is broken when Aggregation is turned on by Mark Conroy (markconroy) (4 February 2026)

Debugging

Frontend

Mark Conroy

Drupal Theming

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