Why can't I type two spaces after the period and let the software figure out how to typeset it? That's how it worked for years in html, then sites like faceboo

Why can't I type two spaces after the period and let the software figure out how to typeset it? That's how it worked for years in html, then sites like facebook come along and use "white-space: pre-wrap" to force the extra space to show up, even though it's not a monospace font. What's the reason for this?