Hi Sandeep,
We had similar frustrations with trying to create a higly customized form. There isn't anything you can do with HTML. Everything will need to be done in CSS and Javascript on the landing page where you choose to place your form.
We had to go to an outside developer to set up
our subscription centre. It involved a lot of time with Marketo Support to get the fields to properly feed back into the Marketo system and properly pre-populate, etc.
We send incredibly incredibly segmented campaigns so we ask for a lot of information from our leads. It was just not possible to organize our fields in a friendly way with the default Marketo form setup, and we had to go the customized route. Depending on your needs, you may need to do something similar.
Good luck!