Why does IE not display the webpage correctly?

A Web page has been written to display both correctly in Chrome and in Firefox, with no warning or error messages.
However, in IE6, 7, 8, how can not be displayed, the following error is indicated. It can be displayed in IE before,
After maybe copied, pasted some code, there is such a strange mistake, have you ever encountered this situation?

Reply to discussion (solution)

IETester do not use this test, and the actual difference is too big

A Web page has been written to display both correctly in Chrome and in Firefox, with no warning or error messages.
However, in IE6, 7, 8, how can not be displayed, the following error is indicated. It can be displayed in IE before,
After maybe copied, pasted some code, there is such a strange mistake, have you ever encountered this situation?

Developing tools with F12

IE tests for some JS will not be compatible with this hint.

Thank you, all of a sudden. There's no way.

