HTML is not the storage format for forms, behind the scenes. Forms are stored in a proprietary rich text format, and conversion between that and HTML is not exact. So there is no "HTML view" of a Notes form.
You can type HTML text onto the form, highlight it, and designate it as "pass-thru" (Menu 'Text > Pass-Thru HTML'). If accessed via the web, the exact HTML you enter here will be included in the HTML sent to the browser. The Notes client will also convert pass-thru HTML to Notes rich text for display, provided this option has been enabled in the form properties.