Not quite "writing a pixel to the screen", but to play around, you can use a <canvas> tag in HTML, like my demos do; you can use SDL, as another poster commente

Not quite "writing a pixel to the screen", but to play around, you can use a <canvas> tag in HTML, like my demos do; you can use SDL, as another poster commented; or you can write some trivial code to write a PPM file to disk (or use something like libpng) to create image files on disk.