[GMcG]

It seems to be the height in
td{
text-align:left;
height:50px;
vertical-align:middle;
padding:15px;
height:50px;
}

I have checked it in an old Internet Browser (Version 5) which is not so tolerant as new browsers.
The first version shows your original, the second is without the height.
I suppose that the height is meant as a maximum. So there is too much content for this cell and the text is shown in layers.

By the way: Calibre will create a new stylesheet when converting. It is completely different from yours. See example 3.

George

Edit: In Chapter 6 there are some sentences closing without a point at the end (!).