Tailwind CSS v4 Migration: Fixing "@apply", "Unknown Utility Class", and Oxide Build Errors

This is a technical troubleshooting guide for developers whose builds break after upgrading to Tailwind CSS v4, covering @apply failures, "Cannot apply unknown utility class" errors, legacy config bridging, framework-specific fixes (Next.js, Blazor, Vue, Svelte), and VS Code "unknown at-rule" warnings. Readers land on it from error-message searches (the top query relates to VS Code) while trying to get a broken build working again; the supplied content is partial, so coverage of later sections is inferred from the headings only.

Front-end and full-stack developers mid-upgrade to Tailwind v4, with the largest audience in the UAE, searching to fix a specific build or editor error.

A mention would suit developer tooling such as code editors or IDE extensions, hosting and deployment platforms, CI/build services, or developer courses and documentation tools aimed at web developers. It would read most naturally near the TL;DR quick fix or the VS Code configuration section, where readers are actively fixing their setup; consumer or non-technical brands would fit poorly, and traffic is small (5 organic clicks in 28 days) with no AI answer impressions.

Summarised from the live page by AI. Traffic figures come from the publisher's own Search Console, last 28 days.