Change HTML DOCTYPE in APM Logon Page

Hello,

I'm using APM with custom CSS. On Logon Page, the APM module uses the following DOCTYPE as first line in HTML code- this breaks some CSS features on IE 9 and IE 11 (IE goes into Quirks mode)

To have my CSS running correctly, I need the DOCTYPE like I have after logging in to APM: http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">

Do you have a recommendation to fix that?

Thanks in advance.