That website actually has some errors on it. Quoted from the site
"If Prince Charles had become King during their marriage then she would have been his consort with the title Queen Diana. They divorced in 1996 and she died in a car crash in Paris on 31st August 1997. Although popularly known as 'Princess Diana' this was not her title - because she became a princess by marrying Prince Charles her title was Princess Charles Philip Arthur George, Princess of Wales, or more simply Diana, Princess of Wales."
In her life her tile was HRH the Princess of Wales. The Title Diana, princess of Wales was agreed when she divorced in order to remove the HRH.
Apparently the current intention is that she will become Princess Consort. This taken from the official Prince of Wales website
"Will The Duchess become Queen when The Prince becomes King?
As was explained at the time of their wedding in April 2005, it is intended that The Duchess will be known as HRH The Princess Consort when The Prince of Wales accedes to The Throne."