Thank you for this long and informative reply!
I am happy to get such friendly feedback,I will try to improve!
the way youve outlined your heart with multiple outlines creates an effect called banding, which in pixelart makes your image look jagged or blurry in some cases. You can use a technique called anti-aliasing (AA) to get rid of that effect. Heres an example:
if you have a background, it looks smoother to AA the foreground object into it than use a solid outline alot of the time. If youre using a transparent background its better to use a solid outline because you dont know if the veiwer's background color will make the AA look wrong (the AA color should be between the color of the object and the background in terms of hue value and saturation)
of course, you can use solid outlines if you have a background, too, but you can smooth the inside of the outlines by using AA instead of banding. Hope this helps explain some of the technical side of pixel art for you.