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Double space or single space?

Should periods at the end of sentences be followed by one space or two? Two used to be popular, but seems to have fallen out of favor for reasons other than being proven worse (largely due to software shortcuts in word processors & HTML). I looked into the available references in Wikipedia & within the cited papers.

The available research on the issue turns out to be scanty, low-quality in the typical HCI way, and several cited papers were unobtainable.

Further, there are still no good ways to implement double-spacing in HTML/CSS. (Because there is no concept of ‘sentence’ in HTML, it’s hard. The suggested approaches are to either wrap sentences in <span> elements, which would increase the overhead of already over-burdened DOMs, or to insert Unicode spaces into the text, which is error-prone.) So there would be little point in running an A/B test: it is highly unlikely to yield a measurable benefit, and even if it did, the cost of implementation would probably exceed that.