Checking the source code has become our usual practice now. But it doesn’t seem to do any good in this level in the beginning but later you will find that level 7 is really easy, only a look at the right place is needed.
There is a little clue given to us by the coders of hackertest.net in the name of the file we are on i.e. included.htm, so one may consider searching for some include statement or tags. Soon you find an image file included in the body tag.
bg attribute stands for background. When you open the image using directory traversal you will be able to find out the needed username and password in some corner.
Hiding data in images is knows as stenography. Look for it on wikipedia for more information.
For those lazy champs, Username: phat Password: jerkybar3.