Typing Habits vs HTML
You can tell the old schoolers who were taught in typing class to double-space after a sentence.
Several thousand typed pages later, I had to un-learn that.
Strict HTML rules say that browsers are supposed to collapse all "white space" - tabs, spaces, and line feeds - into one space (or linefeed, whichever). So you shouldn't even see double-spaces or multiple linefeeds.
But, because browsers are built to accommodate poorly-written HTML, you will see double-spaces and multiple linefeeds, anyway.
But I was an early convert to "proper" HTML, and learned to single-space after sentences, and never looked back.