I appreciate your swift reply.
About item 1,2,3,4,7 and 8, I understand.
About item 5 :
Fig. MathJax_2vs3_01.jpg is the hardcopy from my computer.
(Upper layer: Ver.2 / Lower layer: Ver.3)
The difference is very small as you wrote.
But I prefer the arrow slightly moved to the right.
Furthermore, small difference of level on the center of arrow would be removed.
About item 6 :
Fig. MathJax_2vs3_02.jpg is same as above.
(Upper layer: Ver.3 / Lower layer: Ver.2)
The difference of line height is large than your hardcopy.
Particularly, the bottom margin of the formula is larger.
For your reference, my computer is windows 10 machine.
Any browser (Chrome, Firefox, Edge) brings the same result.