Estimating the x-height of a font
This page can be used to get an estimate of the relative x-height (aspect ratio) of a font. Specify a font and a guess of x-height in pixels for a font size of 1,000 pixels. The vertical bar to the left of the letter x (displayed in that font size) will then occupy the height you specified. The initial guess is 500. Once the vertical bar is as tall as the letter, you have found a rather accurate estimate for the x-height of the font. For example, if height 466 gives the match, then the relative x-height of the font is 0.466.
If you specify a font that does not exist in your system (e.g., mistype the font name), the x appears in the Courier font, to make it easier to recognize that there is a problem.
Although the results seem to indicate that the CSS 2.0 specification was in error in this matter, it is possible that its figures apply to some special circumstances. Depending on the implementations, there might be different relative x-heights for a font in different sizes, even though this may sound illogical. Perhaps results are intentionally different, because small font sizes impose their own requirements, so that the x-height is larger than in other fonts, to keep all letters minimally legible. I am indebted to Bruno Fassino for the observation of such possibilities as well as other notes on this theme.