The demo under Basics\HTML shows how Delphi component instances can be mapped on HTML elements via the ElementID. So, there is not much more to it than knowing the HTML element ID and setting this to the control.ElementID property. Of course, the control type must match the HTML element type. A TWebEdit maps to a HTML INPUT element for example. A SELECT to a listbox or combobox, etc...
The ElementClassName is used to set the main CSS classname for a control. See the demo under Basics\Bootstrap for the use of Bootstrap CSS with web controls.
There is no direct HTML equivalent element to a dashboard control, so it is normal that such complex control cannot be mapped to a (simple) HTML element.
But how to insert a dashboard component inside an html page.
I'm not sure I understand your question. If you drop the component on the form at design-time, it will be automatically displayed in the form HTML page.
What else specifically do you want to do?