Using IE and the plain contenteditable element, pressing keyboard shortcuts at the beginning of the paragraph:
Ctrl + B
, Ctrl + U
and Ctrl + I
works as expected and results in correct formatting:
<strong><em><u>text</u></em></strong>
In CKEditor the result is different (only the last style is applied, as reported in TC):
<em>text</em>
You can compare the results in CKEditor and contenteditable element using the attached replacebyclass sample.
I'm adding this comment, because I initially thought that IE is unable to apply more than one style.