Fix in original font size calculation

There are two bugs in font size calculation:

1) If font-size style was changed for body element - "rem font size" will take changed value but Root Em value should be used. For example:

body {
  font-size: 0.75em;
}

in this case all modern browsers will render rem with 16px, but script will use 12px.

2) Font size calculation doesn't work when font size is in em or pt. Raw em/pt values are using which is completely incorrect