How to draw a single pixel using `gl.POINTS` on webgl?

I'm trying to draw a single pixel using WebGL. I'm using gl_PointSize = 1.0, and gl_FragColor = vec4(0.0, 0.0, 0.0, 1.0). I expected single black pixels. Yet, this is how my points are rendered:

That is, what I get is gray dots covering an area of about 3x3. How do I get actually single pixels?