htmlfile: Unexpected call to method or property access

You could set CSS properties on the standard button class[^] (using the Style property for inline css (IIRC) or the Attributes property for setting the class). You could also create a class inheriting from Button and override the Render method, you can then consume the Click event as you would with a standard Button.

For more customisation, you could create your own custom Button class that inherits from WebControl and override its Render method:

public class RoundedButton : System.Web.UI.WebControls.WebControl
{
    protected override void Render(System.Web.UI.HtmlTextWriter writer)
    {
        // render custom html here using the HtmlTextWriter
    }
}

Check this[^] out for information about developing custom server controls.

There is also a HtmlButton[^] class which you could use in a similar fashion.