I have figured that 3 times out of 4, the height of the document root
element will cause a fractional pixel situation and there will be a 1px
difference between the test and the reference file in Chrome 50+.
I tried to make a simple reference files but the layout is quite
demanding, has many constraints and is not easy to achieve.
The slot numbers missing (003, 004, 005, 007, 008, 009, 011, etc..) are
in case we need to create the same tests with vertical-lr, sideways-rl,
Each test has an URL to its equivalent 'horizontal-tb' test in a
<!--comment--> in the <head>.