The behavior of Firefox looks like a bug. The script doesn't add any white space characters that should be taken into account by white-space property. Interestingly, the same issue occurs for body > div::before { content: '' } instead of prepending empty string with the script.

That's super weird, and it's definitely a bug. Different DOMs that output the same source code should not result in different CSS output, ever.

@SelenIT ::before { content: '' } is a little different, it causes the ::before pseudo-element to exist, and empty inline boxes count as content that causes a line box to exist.

empty inline boxes count as content that causes a line box to exist

They do, but shouldn't they be treated as "not existing for any other [than positioning out-of-flow descendants] purpose" phantom line boxes?

@fantasai If I understand correctly, you mean that

• An empty text node generates no text run.
• A ::before or ::after with content: '' generates an empty text run.

Is that correct?

@Loirooriol A ::before or ::after with content: '' generates a box containing nothing.

@SelenIT That depends on if they are assigned any other styles. :)