Why is RenderingContext.drawElements clearing the screen before it draws?

Am I correct when I assert that a call to RenderingContext.drawElements is NOT supposed to clear the screen before it draws?

Consider these screenshots that show me stepping across a call to drawElements. The code in the screenshots is too hard to see, yet it illustrates nothing more and nothing less than a debugger step across the RenderingContext.drawElements function with the object already drawn being erased.