The following table identifies which features are supported by the CrmEntityFormView control that renders the entity form. Web Form and Entity Form also use this control programmatically to render forms.

Feature Supported?
All CRM Field Types (except PartyList) Yes
All CRM Field Validation Rules Yes
Multiple Column Layouts Yes
Web Resources (Html and Image types only) Yes
CRM Label Language Translations Yes
Header & Footer No
Sub-Grids (Configure with Metadata) Yes
Notes Yes
IFRAMEs No
CRM Form Scripting No
Lookup Related Records Filtering* N:1 Only
Lookup Default View* Yes
*Creating a new record via a Lookup within a Form is not supported on the portal at this time (04/2016)
A form that contains features that are not supported will not prevent the form from rendering in the portal, those features simply will not be present (N:N lookup related records filtering is not supported and will cause an error). 

For more information regarding the creation of entity forms and customizations in CRM please refer to the documentation on MSDN by visiting the following URL http://msdn.microsoft.com/en-us/library/gg309646.aspx