I don't think of this as a secure or convenient method. OP, you should look at offline options.
You can use this method offline as well, you can download the page. The only real question is the algorithm, I have not analyzed this specific page so I cannot vouch for it, but I bet that is a port of ninja's code
I trust Ninja's generator 100% (bitaddress.org)
I use bitaddress.org for thousands of dollars worth of bitcoins, but only use bitaddress.org on a computer who has never been online.
But I am not too comfortable with the random generator, so I always input my own random number, personally I like to use a 99 digit base6 number.
That give such a warm fuzzy feeling, completely secure