An email I had viewed with the HTML Renderer had a dodgy font family.
The blockSource had the value "font-family: helveticaneue,;"

The CssParser.ParseFontFamilyProperty method initially extracts the font "helveticaneue", but fails to find it.
It then tries to extract the next font by starting at the index of the comma, and increments the index by one to look at the next character.

Since the comma is the last character, it causes an IndexOutOfRangeException.
There needs to be a check to make sure that the index is never equal to or greater than the length of the property value.