Somebody asked on the Vintage Computer Forums about what the CGA aspect ratio is supposed to be. The answer is usually given as 4:3 (pixel aspect ratio of 5:6), but I was inspired me to find out what the relevant standards say it ought to be, exactly.
The relevant standard in question is SMPTE 170M - composite analogue video signal (upon which CGA is based). This gives an aspect ratio of 4:3, but that is for the full composite picture which 242.5 lines rather than the CGA's 200. The width is given in terms of timings - 63.556 microseconds per scanline total minus 1.5+9.2 microseconds for the blanking period, with a tolerance of +0.3/-0.2 microseconds, so between 52.556 and 53.056 microseconds altogether. Since the full horizontal period consists of 455 CGA low-res pixels horizontally, the full NTSC active area is the equivalent of (376.25-379.83)x242.5 CGA pixels. Re-arranging, that gives us a screen aspect ratio for CGA of between 1.362 and 1.375 - slightly wider than the usually quoted value.
However, no TV or composite monitor of the time was manufactured to have aspect ratio tolerances as precise as 3% - 4:3 would have been well inside the error bars.