A single CNTL-Minus text becomes unreadable, because line-height == 10px, so with an 18px font, a 10px line-height causes text to flow ever itself on every line.
Take a look at the calculations for font-size + line-height during resize operations.
Look for size 10px through your CSS or however you're resizing occurs.
Provide an actual URL for testing.
Likely someone can point out a fix for you.