How can I make blend edges with shaders

How can I make blend edges with shaders

let me introduce my answer.
This is triagnle rendered with webgl. Well it is a little enlarged ...
And this is triangle, which I want to have:
So Im looking for some shader, that will be able to blend edges of
primitive triangle. I have an idea how to realize one, but Im probably not
good enough to write it yet.
My idea is something like: Based on position of 3 vertices calculate for
each fragment, how much does primitive cover pixel, and then set up
transparency of this pixel based on calculated information...
I can get 2D coordinates from vertex shader and use them in fragment
shader. Now I probably want to use gl_FragCoord.xy or gl_PointCoord.xy and
calculate % pixel cover, but I not able to compare these values (it seems
that units are different, I compute miles with milimetres and also 'point
zero' is somewhere else for these vectors), so I can't calculate final
transparency value.
Can anyone help me please? Just turn me correct way.