Hi Naga,
When I originally checked the situation in 2012, it was all but trivial to overwrite the restriction set by SAP. Anyhow, meanwhile there is a possibility to implement a workaround which uses a "half way implemented" support by SAP: Re: Portal Login Page in IE Standard Mode
My last comment here was in SAP's direction, as the reason for the restriction, listed in note 1697320, is no longer valid. So the restriction should be removed in general.
I would advise to create an OSS message asking for this, if you are planning to implement an HTML5 page as portal login page. It makes much more sense that SAP removes the general restriction now instead of customers running individually for a workaround. Note 1697320 should be updated and the half-way implemented workaround should be accessible a bit easier...
Best regards
Detlev