This WebSharper code draws a black rectangle on the hole canvas surface and then clears a 100px X 100px rectangle starting from the coordinates (50, 50).

ClearRect.fs

module ClearRect =
 
    open IntelliFactory.WebSharper
    open IntelliFactory.WebSharper.Html
    open IntelliFactory.WebSharper.Html5
 
    [<JavaScript>]
    let main() =
        let element = HTML5.Tags.Canvas [Text "Fallback content goes here."]
        let canvas  = As<CanvasElement> element.Dom
        canvas.Height <- 200
        canvas.Width  <- 200
        let context = canvas.GetContext "2d"
        context.FillRect(0., 0., 200., 200.)
        context.ClearRect(50., 50., 100., 100.)
        element    

    type Control() =
 
        inherit Web.Control()
 
        [<JavaScript>]
        override __.Body = main() :> _