I had this problem once.

I added the Paddle button to my online course page, so people could click the “Buy now” button and the nice Paddle popup would show up.

The popup has a loading indicator, a circle with a spinner inside, and there was an issue: when clicking the “Buy now” button, the spinner indicator was not centered inside the circle, as show in this gif:

I didn’t really know what was causing the issue, so I was thinking how to solve it.

The principle I use in these cases is bisect. I also call it divide et impera.

I opened the DevTools and moved to the Sources panel, which showed all the files loaded in the page. I searched for one of my CSS files, as my intuition was that the page CSS was interfering with the Paddle own CSS rules.

So I removed all the content of that file.

Chrome automatically changes the appearance of the page when you alter the CSS files in the Sources panel, so I was able to check and see that the spinner was now working fine!

So, one of the rules in that CSS is the problem.

How do I find out which line?

The file has 312 lines. I select from line 150 to 312 and delete it. Try again. The problem is still there, so it must originate from the first 149 lines.

I hit cmd-Z (undo) to put back those lines I removed, and delete lines 70-149.

The problem is gone, so the problematic line is in there. I hit cmd-Z again to restore the lines I deleted.

And so on, you got the idea. Rinse and repeat until you find the line that gives you the problem.

Lessons in this unit:

0:	Introduction
1:	How to center an element with CSS
2:	CSS Border inside the element
3:	What are good CSS Breakpoint values for Responsive Design?
4:	▶︎ How to debug CSS by bisecting
5:	How to disable text selection using CSS
6:	How to put an item at the bottom of its container using CSS
7:	CSS, how to select elements that do NOT have a class
8:	How to stick an element on the bottom of the page with flexbox
9:	How to apply padding to multiple lines in CSS
10:	Making a table responsive using CSS
11:	CSS url()
12:	How to make an element smaller or bigger with CSS
13:	Customizing visited links
14:	Fix extra space after inline element
15:	How to create a sidebar that’s sticky but also scrolls
16:	How to embed YouTube videos using the correct aspect ratio
17:	Responsive pre tags in CSS
18:	Responsive YouTube Video Embeds
19:	How to remove all CSS from a page at once
20:	How I added Dark Mode to my website
21:	How to add a simple dark mode