How come CSS doesn’t recognize the British spelling of ‘center’ as ‘centre’?
The people who created CSS were most likely American. I’m sorry.
Good question, though!
As @naomipinkflower said, the spellings are US English.
After a while, you’ll fall into the habit of spelling it that way whilst working on HTML/CSS.
(I frequently run into a similar problem, where I’m working on something through a CLI and the command uses a
z where I naturally expect to use an