Maybe "best effort" is a misnomer, but it's often used to signify a general attempt without guarantee, rather than the absolute best effort possible. Like a delivery service making a "best effort" to complete delivery in 2 days, but there are no guarantees compared to the more expensive overnight service, or UDP vs TCP as far as packet guarantees.
In this case it seems they make a "best effort"(not guarantee) to offer uniform document display across browsers and OSes, which suggests they may only test the most popular browsers and OSes, and edge cases like Firefox on macOS may not get much love.
https://en.wikipedia.org/wiki/Best-effort_delivery