HR element lost CSS color if has ID attribute

I tested this issue on FCKeditor v2.6.5 and comfirmed that it is really a bug.

I added following CSS to the file fck_editorarea.css :

hr
{ border: red dashed 1px; color: red; }

Then, I click the Horizontal Line button of FCKeditor, I can see that a line in RED color appeared.

Next, I switch to HTML mode and added ID attribute to this HR element, the final code is:

<hr id="system-readmore" />

Now, I return to WYSIWYG mode, I noticed that this horizontal line dissappeared ! It is just invisible !

I switch to HTML mode again, ok, the code is not lost.

So, why the HR element lost CSS color if I add ID attribute to it?

I am sure this is a new bug, hope someone can fix it.

Thanks.