9pt == 12px on the web. You can check that in Web Inspector in computed styles.

Alternatively:

  1px = (1/96)in
  1pt = (1/72)in = (1/72)*(96/72)*(72/96)in = (96/72)*(1/96)in = (4/3)*1px = (4/3)px
  9pt = 9*1pt = 9*(4/3)px = 12px

https://developer.mozilla.org/en-US/docs/Web/CSS/CSS_Values_...