Every front-end developer who has tried to bring a custom HTML/CSS mockup into WordPress has experienced the same quiet heartbreak. In the browser, the static HTML file looks razor-sharp. Ten minutes after moving it into a page builder or block converter, the navigation height is off by 6 pixels, the pricing table borders have vanished, a CSS Grid has collapsed into a single column, and half the inline SVGs refuse to scale.

Why does that happen—even when you copy the exact same CSS file into WordPress? The answer lies in DOM wrapper pollution and cascade order .

1. How Extra Wrappers Break Modern CSS

Modern CSS relies heavily on parent-child relationships: Flexbox, CSS Grid, direct child combinators ( > ), and adjacent sibling combinators ( + ).…