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.