Alexis M. answered 10/28/19
WordPress Expert Specializing in Web Design WITHOUT Code
I think the main reason people don't indent the first sentence in a paragraph is because websites are to be treated differently than Word documents.
It can be easy to initially use the same techniques you may have learned to write essays or letters because you are still writing large chunks of text on websites and in essence still "presenting" that written content as well.
But there are different rules for content layout when it comes to web design. One would be not to indent the first paragraph. Another really important one would be to break up paragraphs into only 2-3 sentences. This makes things much more easy to digest when people are reading things on the internet.
Let me know if this is helpful.