Here is a slowly rotating checkerboard.

  uniform float time;
  varying vec2 vUv;
  varying vec3 vNormal;

  vec2 rot(vec2 uv, float angle) {
    float c = cos(angle);
    float s = sin(angle);
    mat2 rot = mat2(s, -c, c, s);
    return uv * rot;
  }

  void main(){
    vec3 black = vec3(0.0);
    vec3 white = vec3(1.0);
    vec3 color = black;

    vec2 uv = vUv;
    uv = rot(uv, sin(time) * 0.05);

    float boardSize = 24.0;
    uv = uv * boardSize;

    vec2 gridId = floor(uv);

    if (mod(gridId.x + gridId.y, 2.0) <= 0.01) {
      color = white;
    } else {
      color = black;
    }

    gl_FragColor = vec4(color, 1.0);
  }