I ran into a problem when working with PHP buffer. The essence of the problem: when you connect an HTML file in which there is a CSS file connection, CSS is not displayed on the page. I double-checked the code, and through the inspector of the elements, too - in the browser this CSS file is opened by reference without any problems, the paths are correct. I was looking for a problem with Google but I didn’t find anything.
ob_start();
require $filePath;
ob_get_flush();
and required files.
<link rel="stylesheet" href="<?= ROOT ?>css/main.css">