WordPress has this problem, sometimes. I tried to sign in the backend of the WordPress website. It took a little bit longer to show the page without CSS or JS.
It looks like below:
So I open the Web Developer Tools of the browser Firefox. Check the Network.
Refresh the page.
There is one 500 error on the site.
/wp-admin/locad-styles.php?**********
It is a script to combine the styles into one file. Then it is broken.
The solution I found on Google is to add a configuration line into the wp-config.php file.
Open the wp-config.php file.
Add the following before the require_once.
define('CONCATENATE_SCRIPTS', false);
Then, refresh the Admin page. It looks OK.
All the CSS files are loaded individually.
The real reason, I believe, is because the Plugin or Theme has a small compatible issue with the WordPress program.
