I was having a conversation with several graphic artists, typography experts all, who were of the opinion that one and only one space at the end of a sentence was preferable to two. It's something that I never truly thought about as it not convention on HTML pages. (Although I do remember when clients asked me how to create the extra space.)
Apparently the convention of using two spaces after a period came into play with the adoption of manual typewriters. Typewriters used monospaced fonts that put a lot of space between letters and words. The extra space was necessary in order for the reader to more easily visualize the end of sentences. This is not necessary anymore as we're using proportional typesetting. Instead of helping the eye transition to the new sentence big holes are being put in the middle of the text adversely affecting the transition.