[raegis]

I just tested both on the text "Look Dick. See Jane. Jane run home. I says you go home to. They eats dinner." LanguageTool does what I would expect. Harper does not. However, both whine about two spaces after a period.

Edit: Alas, Hacker News also removes the extra space after periods.

[dreamcompiler]

Extra space after periods is never correct with proportionally-spaced fonts, which is why all browsers remove it by default.

Two spaces after periods is a kludge invented for typewriters that had monospaced fonts and touch typing teachers need to stop teaching it in the modern era where most writing uses proportional fonts.

[dkga]

Curiously, it‘s also a tell-tale sign that a North-American typed the text.

[codesnik]

browser rendering does. You'd need white-space: pre-wrap rule to retain double spaces.

[chilipepperhott]

Harper will detect those errors in its next release.