Everything in typography is about being subtle, and styling the separation between paragraphs is no different.
Denoting a new paragraph should hardly be noticable to the reader, and first-line indents are a good way to achieve that. The first paragraph doesn’t need to be indented.
Spacing is the other way to set off a new paragraph. You can do this with
the bottom-margin CSS property.
We find that using a bottom margin makes it a little easier to scan a web page, while indents give more of a narrative or newspaper-y feel.
The purpose of a first-line indent or bottom margin is to indicate to the reader that, well, you’re starting a new paragraph.
Readers aren’t stupid, and they don’t need two signs that a new paragraph is happening. That’s redundant.