WebGL: unexpected effects in canvas when setting alpha lower than 1.0

Question

It seems like canvas' alpha depends on the CSS Background color (tested on Chrome 60 and Firefox 47).

body {background-color: #000000} /* or any other parent DOM element */

Setting Canvas Color to red with 10% alpha

gl_FragColor = vec4(1.0, 0.0, 0.0, 0.1);

has this result:

-> I expect red to be 90% darker

I got the same result after puting alpha to 100%:

gl_FragColor = vec4(1.0, 0.0, 0.0, 1.0);
return;

-> it looks correct

-> it looks like red part is more than 10%

Is there any Color Blending mode, which I'm not aware of?

(These tests were the same on Firefox and Chrome)

this has been answered many times. I fact the linked answer was on the right "related questions" links — gman, Sep 07 '17 at 09:52
others: https://stackoverflow.com/questions/12273858/how-to-make-webgl-canvas-transparent, https://stackoverflow.com/questions/38636837/math-formula-for-color-and-alpha-in-webgl, https://stackoverflow.com/questions/39341564/webgl-how-to-correctly-blend-alpha-channel-png/, https://stackoverflow.com/questions/29217189/alpha-blending-in-webgl-works-not-correctly, https://stackoverflow.com/questions/35724317/webgl-colors-and-alpha, https://stackoverflow.com/questions/20362023/webgl-why-does-transparent-canvas-show-clearcolor-color-component-when-alpha-is — gman, Sep 07 '17 at 09:58
cont... https://stackoverflow.com/questions/20362023/webgl-why-does-transparent-canvas-show-clearcolor-color-component-when-alpha-is — gman, Sep 07 '17 at 09:59
It solves my problem. I'm ashamed to not have found these questions. Thank you for your fast help. You are really everywhere where someone types in "WebGL" :) — Tobias, Sep 07 '17 at 10:07