Draws a square in a fragment shader (unoptimized implementation).

square.frag

#ifdef GL_ES
precision mediump float;
#endif

uniform vec2 u_resolution;
uniform vec2 u_mouse;
uniform float u_time;

void main(){
    vec2 st = gl_FragCoord.xy/u_resolution.xy;
    vec3 color = vec3(0.0);
    
    float left = 1.0 - step(0.1,st.x) + step(0.2,st.x) + step(0.9,st.y) + 1.0-step(0.1,st.y);
    float bottom = 1.0 - step(0.1,st.y) + step(0.2,st.y) + step(0.9,st.x) + 1.0-step(0.1,st.x);
    float right = 1.0 - step(0.8,st.x) + step(0.9,st.x) + step(0.9,st.y) + 1.0-step(0.1,st.y);
    float top = 1.0 - step(0.8,st.y) + step(0.9,st.y) + step(0.9,st.x) + 1.0-step(0.1,st.x);
    
    float pct = left * bottom * right * top;    
    
    color = vec3(pct);

    gl_FragColor = vec4(color,1.0);
}