In Chrome.

Could be due to font-smoothing.

With -webkit-font-smoothing, there are 3 possible values:

• none – turn font smoothing off, show the text with jagged sharp edges.
• antialiased – smooth the font on the level of the pixel, as opposed to the subpixel. Switching from subpixel rendering to antialiasing for light text on dark backgrounds makes it look lighter.
• subpixel-antialiased – on most non-retina displays this will give the sharpest text.

Usually the second one does the trick of returning font to normal weight.