Here are guides on writing shaders for flixel and openfl.

And to make sure that the shaders works on as many devices as possible.

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Always have numbers be 0.0 not 0. or .0

Don't use Switch cases, use if statements instead

Use floats where its expected, such as vec2,3,4

Don't use "texture" use "flixel_texture2D" or "texture2D"

(These might work, but its not recommended)
Don't use ivec2,3,4
Don't use bvec2,3,4
Don't use uvec2,3,4
Don't use uint

Don't name variables starting with "gl_"

Don't have default values for uniforms, set it in the constructor if a class, or after loading the shader

Don't use <number>u, like 8u
Don't use the variable name "input"
Don't use the variable name "sample"

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Common Practice:

Use texture2D for camera shaders, and FlxGame shaders, and flixel_texture2D for everything else
flixel_texture2D applies the .colorTransform.
texture2D doesn't apply it.

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Extra:

Use tabs for indentation