Okay, some Guys will know what i mean and edit my Question but they did it wrong.
A better explanation:
You have a contenteditable div with the text This is a Test String..
If you use now the execCommand('underline') on Test String you get This is a <u>Test String</u>
if you use now the execCommand('strikethrough') on is a Test you get This <s>is a <u>Test</u></s><u>String</u>, THIS is correct.
So, in HTML5 <u> and <s> are obsolete.
For the first execCommand you can use the surroundContents() with a <span style="text-decoration:underline;">. If you now use the surroundContets() for the second execCommand you receive the BAD_BOUNDARYPOINTS_ERR.
The Thing i want is a Function which works like the execCommand in this case but with functions where i can define with witch HTML-Tag the String will wrapped… (It should be intelligent in the case if there is any overlapping…)