The font validator checks the left and right sidebearings on some of your glyphs. We will look at your font design to ensure that spacing is consistent.
A common behavior for novice font designers is lining up a letter in the left-hand side of the glyph box, with all the space on the right hand side. This is not the correct way to space a font. For the designer, this inevitably causes spacing issues in the fonts themselves, making them very difficult to kern. This also causes issues for customers; when a customer goes to use the font in an existing layout, everything will shift in unexpected ways because the spacing is not distributed properly and is abnormally set up. It also causes subtle cognitive dissonance when customers type with the font, as the letters are not centered in the glyph boxes.
Glyphs should be roughly centered within glyph boxes. Of course, per basic type design principles, the space on either side of glyphs are not always exactly the same. Learn more about the principles of spacing in our common errors in type design series.
If your font is an italic or script typeface, please disregard this check. Italic or script letter's relationships with the glyph box is very different than upright fonts, and will almost always be uncentered.