The ‘height’ paremeter creates the needle shape of dimension 200 pixels by 4 pixels
The ‘background’ parameter creates a linear-gradient(black 50%, transparent 50%). This defines the background color of the needle shape. Linear-gradient is a really useful CSS property that has many uses in canvas apps. Here I’m using it to color the needle black for its upper half and transparent for it’s lower half. The reason for this will become apparent shortly
The ‘transform’ parameter uses the transform: rotate property to rotate the shape depending on the value of a Slider control called sldDial. The Value of the Slider control is injected into the CSS using concatenation
The ‘position’ and ‘top’ parameters define where the needle is positioned within the HTML Text box