In my understanding, internal CSS is code written directly within the HTML file (inside the <style> tag). It can be used for small projects, single-page websites, or quick testing. On the other hand, external CSS is code written in a separate .css file and linked to the HTML file. This approach is ideal for multi-page websites or larger projects because: It is reusable across multiple pages. It is easier to maintain and update. It improves website performance (files can be cached by the browser). For most modern websites, external CSS is the preferred approach as it ensures consistency, simplifies maintenance, and enhances performance. Internal CSS is generally not recommended by search engines for websites, because of this basic rule, as it can negatively impact SEO and overall performance. I did not find any other modern plugin using internal CSS than brizy. Given that Brizy is a large and widely-used plugin, I believe it should be aware of this basic rule, as search engines favor external CSS for better website optimization.