End of SAP Quirks Mode Support for IE
Starting from NetWeaver 7.52 and/or SAP Kernel 753 and/or SAP_UI 7.52 used in lower NetWeaver AS ABAP versions, Quirks Mode (IE 5.5 rendering mode) of Internet Explorer is no longer supported by SAP for all SAP UI technologies (the SAP release date is planned for the end of Q3/2017) and cannot be used any longer (usage blocked).
For more information, see also SAP Note 1672817 – Browser: Microsoft Edge and Internet Explorer Support Policy Note and SAP Product Availability Matrix (PAM) for NetWeaver –> “Essential Information” (e.g. PAM for NW AS ABAP 7.51).
See also portal related information https://blogs.sap.com/2017/04/02/bye-bye-quirks/.
See also the external Microsoft article 4014991 (external link): Slow rendering in Internet Explorer 11 for websites displayed in Quirks Mode.