One Space

As a general typographic rule, only a single space is required between sentences.

Many people have been incorrectly taught to end a sentence and hit the space bar twice. Why? Because it's easier to read sentences when they're more spaced out? No — in fact, the opposite is true.

This horribly bad habit originated in the days of blackletter and gothic typefaces. Type was dark and heavy. Printing methods were literally medieval. Large blocks of type may have benefited from the extra breathing room.

Thankfully, those days are long gone.

Today, the extra space leads to tiny rivers of emptiness flowing through your paragraphs. It's much easier to read an evenly colored paragraph. In typography, color refers to the denseness of the page. An evenness of color helps the reader's eye bounce along evenly. Add an extra space between every paragraph and the eye is jerked along, constantly stopping and starting, or caught drowning in a spacious typographic river.

In the nineteenth century, which was a dark and inflationary age in typography and type design, many compositors were encouraged to stuff extra space between sentences.

Generations of twentieth century typists were then taught to do the same, by hitting the spacebar twice after every period [full stop]. Your typing as well as your typesetting will benefit from unlearning this quaint Victorian habit. As a general rule, no more than a single space is required after a period, colon or any other mark of punctuation.

—Robert Slimbach, Elements of Typographic Style

Luckily, the web makes adding two spaces much more difficult. Multiple spaces or tabs in HTML are treated as a single space. To actually print two spaces in HTML, the developer need adds special code for a non-breaking space. That makes it even more offensive when people add an extra space on the web.