Data Limitations

What the records here do not cover, and why

NFL

Games from 1920 to 1998 come from FiveThirtyEight's game results, which have no week numbers. Those seasons' schedules are grouped by date instead, and say so.
The same results mark a playoff game as a playoff game and name no round, so a championship before 1970 comes from Wikipedia's list of NFL champions rather than from the games.
Head coaches before 1999 come from a list of tenures by season, not by game. A coach changed in mid-season appears as two coaches in that season, and no coach before 1999 is marked as an interim: the Packers count 16 head coaches here where the club counts 15, because Hugh Devore's 1953 stand-in is counted as a regular.
From 1999, schedules, results and coaches come from nflverse, game by game.

The season being played is shown from ESPN's public scoreboard, which can briefly differ from the league's own record of a game.
No box scores or player statistics are loaded. Records that need them — no-hitters, cycles, earned run averages, a player's game — are left out rather than shown empty, and there are no player pages.
Each league's day-by-day and week-by-week scores come from ESPN's scoreboard, which reaches back years rather than to each league's first season. "Every season, from the record" under that list has the whole history.