Is it impossible to fit a large html table into a small space?

Display the text with an ellipsis after the 10th character and display a hint text with the full text when the user moves his mouse over it…

An ellipsis are three dots… I use them quite often… Did you see?…

Show each logical row in two actual rows, making them clearly coherent using zebra stripes?

I can imagine situations where that would work, but it's highly dependent on the data.

You could use a horizontal accordion on the table if only one column in a row needs to be expanded at a time. Or, you could do something like this answer (or even this). In the demo, click on a table header to see the result.