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HTML

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HTML preprocessors can make writing HTML more powerful or convenient. For instance, Markdown is designed to be easier to write and read for text documents and you could write a loop in Pug.

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Adding Classes

In CodePen, whatever you write in the HTML editor is what goes within the <body> tags in a basic HTML5 template. So you don't have access to higher-up elements like the <html> tag. If you want to add classes there that can affect the whole document, this is the place to do it.

About the <head>

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Insecure Resource

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CSS

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CSS preprocessors help make authoring CSS easier. All of them offer things like variables and mixins to provide convenient abstractions.

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CSS Base

About CSS Base

It's a common practice to apply CSS to a page that styles elements such that they are consistent across all browsers. We offer two of the most popular choices: normalize.css and a reset. Or, choose Neither and nothing will be applied.

Vendor Prefixing

About Vendor Prefixing

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JavaScript

About JavaScript Preprocessors

JavaScript preprocessors can help make authoring JavaScript easier and more convenient.

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Babel includes JSX processing.

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HTML 

              
                <script id="perlin.frag" type="x-shader/x-fragment">
    #ifdef GL_ES
    precision highp float;
    #endif

    varying vec2 vTextureCoord;

    uniform sampler2D uperm_sampler;
    uniform sampler2D igrad_sampler;
    uniform float time;
    uniform float darkness;

    float fade(in float t)
    {
        return t * t * t * (t * (t * 6.0 - 15.0) + 10.0); // new curve
    }
    float perm(in float x)
    {
        return texture2D(uperm_sampler, vec2(x / 256.0, 0)).r * 256.0;
    }
    float grad(in float x, in vec3 p)
    {
        return dot(texture2D(igrad_sampler, vec2(x,0)).rgb, p);
    }
    float lerp(in float a, in float b, in float c)
    {
        return mix(a,b,c);
    }
    float inoise(vec3 p)
    {
        vec3 P = mod(floor(p), 256.0);
        p -= floor(p);
        vec3 f = vec3(fade(p.x), fade(p.y), fade(p.z));
        float A = perm(P.x) + P.y;
        float AA = perm(A) + P.z;
        float AB = perm(A + 1.0) + P.z;
        float B =  perm(P.x + 1.0) + P.y;
        float BA = perm(B) + P.z;
        float BB = perm(B + 1.0) + P.z;


        return lerp(
            lerp(lerp(grad(perm(AA), p),
                grad(perm(BA), p + vec3(-1.0, 0.0, 0.0)), f.x),
                lerp(grad(perm(AB), p + vec3(0.0, -1.0, 0.0)),
                    grad(perm(BB), p + vec3(-1.0, -1.0, 0.0)), f.x), f.y),
            lerp(lerp(grad(perm(AA + 1.0), p + vec3(0.0, 0.0, -1.0)),
                    grad(perm(BA + 1.0), p + vec3(-1.0, 0.0, -1.0)), f.x),
            lerp(grad(perm(AB + 1.0), p + vec3(0.0, -1.0, -1.0)),
                grad(perm(BB + 1.0), p + vec3(-1.0, -1.0, -1.0)), f.x), f.y),
        f.z);


    }

    void main(void) {

        float u = gl_FragCoord.x * 0.001;
        float v = gl_FragCoord.y * 0.001;
        float w = gl_FragCoord.z * 0.001;
        float noiseCoef = 0.1;
        for (float level = 1.0; level < 5.0; level ++)
        {
            noiseCoef = noiseCoef + (0.1 * level) 
                * abs(inoise(
                    vec3(level * u * 10.0, 
                         level * v * 10.0,
                         time)));
        }
                                            
        noiseCoef = noiseCoef;

        gl_FragColor = vec4(0, noiseCoef, noiseCoef, 1);
    }
</script>
<script id="perlin.vert" type="x-shader/x-vertex">
    attribute vec3 aVertexPosition;
    attribute vec2 aTextureCoord;

    uniform mat4 uMVMatrix;
    uniform mat4 uPMatrix;

    varying vec2 vTextureCoord;
    void main(void) {
        gl_Position = vec4(aVertexPosition, 1.0);
        vTextureCoord = aTextureCoord;
    }

</script>
<canvas id="c" style="border: none;" width="1000px" height="1000px"></canvas>
!

CSS 

              
                * {
  margin:0px;
  padding:0px;
  overflow:hidden;
}
!

JS 

              
                /**
 * Provides requestAnimationFrame in a cross browser way.
 */
window.requestAnimFrame = (function() {
  return window.requestAnimationFrame ||
         window.webkitRequestAnimationFrame ||
         window.mozRequestAnimationFrame ||
         window.oRequestAnimationFrame ||
         window.msRequestAnimationFrame ||
         function(/* function FrameRequestCallback */ callback, /* DOMElement Element */ element) {
           window.setTimeout(callback, 1000/60);
         };
})();

var gl;
var permutation = new Uint8Array([151,160,137,91,90,15,
   131,13,201,95,96,53,194,233,7,225,140,36,103,30,69,142,8,99,37,240,21,10,23,
   190, 6,148,247,120,234,75,0,26,197,62,94,252,219,203,117,35,11,32,57,177,33,
   88,237,149,56,87,174,20,125,136,171,168, 68,175,74,165,71,134,139,48,27,166,
   77,146,158,231,83,111,229,122,60,211,133,230,220,105,92,41,55,46,245,40,244,
   102,143,54, 65,25,63,161, 1,216,80,73,209,76,132,187,208, 89,18,169,200,196,
   135,130,116,188,159,86,164,100,109,198,173,186, 3,64,52,217,226,250,124,123,
   5,202,38,147,118,126,255,82,85,212,207,206,59,227,47,16,58,17,182,189,28,42,
   223,183,170,213,119,248,152, 2,44,154,163, 70,221,153,101,155,167, 43,172,9,
   129,22,39,253, 19,98,108,110,79,113,224,232,178,185, 112,104,218,246,97,228,
   251,34,242,193,238,210,144,12,191,179,162,241, 81,51,145,235,249,14,239,107,
   49,192,214, 31,181,199,106,157,184, 84,204,176,115,121,50,45,127, 4,150,254,
   138,236,205,93,222,114,67,29,24,72,243,141,128,195,78,66,215,61,156,180]);

function initGL(canvas) {
    try {
        gl = canvas.getContext("experimental-webgl");
        gl.viewportWidth = canvas.width;
        gl.viewportHeight = canvas.height;
    } catch (e) {
        return -1;
    }
    if (!gl) {
        alert("Could not initialise WebGL, sorry :-(");
    }
}


function getShader(gl, id) {
    var shaderScript = document.getElementById(id);
    if (!shaderScript) {
        return null;
    }

    var str = "";
    var k = shaderScript.firstChild;
    while (k) {
        if (k.nodeType == 3) {
            str += k.textContent;
        }
        k = k.nextSibling;
    }

    var shader;
    if (shaderScript.type == "x-shader/x-fragment") {
        shader = gl.createShader(gl.FRAGMENT_SHADER);
    } else if (shaderScript.type == "x-shader/x-vertex") {
        shader = gl.createShader(gl.VERTEX_SHADER);
    } else {
        return null;
    }

    gl.shaderSource(shader, str);
    gl.compileShader(shader);

    if (!gl.getShaderParameter(shader, gl.COMPILE_STATUS)) {
        alert(gl.getShaderInfoLog(shader));
        return null;
    }

    return shader;
}


var shaderProgram;

function initShaders() {
    var fragmentShader = getShader(gl, "perlin.frag");
    var vertexShader = getShader(gl, "perlin.vert");

    shaderProgram = gl.createProgram();
    gl.attachShader(shaderProgram, vertexShader);
    gl.attachShader(shaderProgram, fragmentShader);
    gl.linkProgram(shaderProgram);

    if (!gl.getProgramParameter(shaderProgram, gl.LINK_STATUS)) {
        alert("Could not initialise shaders");
    }

    gl.useProgram(shaderProgram);

    shaderProgram.vertexPositionAttribute = gl.getAttribLocation(shaderProgram, "aVertexPosition");
    gl.enableVertexAttribArray(shaderProgram.vertexPositionAttribute);

    shaderProgram.textureCoordAttribute = gl.getAttribLocation(shaderProgram, "aTextureCoord");
    gl.enableVertexAttribArray(shaderProgram.textureCoordAttribute);

    shaderProgram.pMatrixUniform = gl.getUniformLocation(shaderProgram, "uPMatrix");
    shaderProgram.mvMatrixUniform = gl.getUniformLocation(shaderProgram, "uMVMatrix");
    shaderProgram.perm_sampler = gl.getUniformLocation(shaderProgram, "uperm_sampler");
    shaderProgram.grad_sampler = gl.getUniformLocation(shaderProgram, "igrad_sampler");
    shaderProgram.time = gl.getUniformLocation(shaderProgram, "time");
    shaderProgram.darkness = gl.getUniformLocation(shaderProgram, "darkness");
    shaderProgram.mouseY = gl.getUniformLocation(shaderProgram, "mouseY");
}


function handleLoadedTexture(texture) {
    gl.bindTexture(gl.TEXTURE_2D, texture);
    gl.pixelStorei(gl.UNPACK_FLIP_Y_WEBGL, true);
    gl.texImage2D(gl.TEXTURE_2D, 0, gl.RGBA, gl.RGBA, gl.UNSIGNED_BYTE, texture.image);
    gl.texParameteri(gl.TEXTURE_2D, gl.TEXTURE_MAG_FILTER, gl.NEAREST);
    gl.texParameteri(gl.TEXTURE_2D, gl.TEXTURE_MIN_FILTER, gl.NEAREST);
    gl.bindTexture(gl.TEXTURE_2D, null);
}


var neheTexture;
var p_perm;
var p_gradient;
var checker;

function initTexture() {
    neheTexture = gl.createTexture();
    neheTexture.image = new Image();
    neheTexture.image.onload = function () {
        handleLoadedTexture(neheTexture)
    }

    neheTexture.image.src = "nehe.gif";
    checker = gl.createTexture();
    gl.bindTexture(gl.TEXTURE_2D, checker);
    var pixels = new Uint8Array([255,0,0,
                                    0,255,0,
                                    0,0,0,
                                    0,0,255]);
    gl.texParameteri(gl.TEXTURE_2D, gl.TEXTURE_WRAP_S, gl.REPEAT);
    gl.texParameteri(gl.TEXTURE_2D, gl.TEXTURE_WRAP_T, gl.REPEAT);
    gl.texParameteri(gl.TEXTURE_2D, gl.TEXTURE_MIN_FILTER, gl.NEAREST);
    gl.texParameteri(gl.TEXTURE_2D, gl.TEXTURE_MAG_FILTER, gl.NEAREST);
    // gl.texParameteri(gl.TEXTURE_2D, gl.TEXTURE_MIN_FILTER, gl.LINEAR);
    // gl.texParameteri(gl.TEXTURE_2D, gl.TEXTURE_MAG_FILTER, gl.LINEAR);
    gl.pixelStorei(gl.UNPACK_ALIGNMENT, 1);
    gl.texImage2D(gl.TEXTURE_2D, 0, gl.RGB, 2, 2, 0, gl.RGB, gl.UNSIGNED_BYTE, pixels);

    p_perm = gl.createTexture();
    gl.bindTexture(gl.TEXTURE_2D, p_perm);
    gl.texParameteri(gl.TEXTURE_2D, gl.TEXTURE_WRAP_S, gl.REPEAT);
    gl.texParameteri(gl.TEXTURE_2D, gl.TEXTURE_WRAP_T, gl.REPEAT);
    gl.texParameteri(gl.TEXTURE_2D, gl.TEXTURE_MIN_FILTER, gl.LINEAR);
    gl.texParameteri(gl.TEXTURE_2D, gl.TEXTURE_MAG_FILTER, gl.LINEAR);
    gl.texImage2D(gl.TEXTURE_2D, 0, gl.LUMINANCE, 256, 1, 0, gl.LUMINANCE, gl.UNSIGNED_BYTE, permutation);
    var g = new Uint8Array([1,1,0,    -1,1,0,    1,-1,0,    -1,-1,0,
                            1,0,1,    -1,0,1,    1,0,-1,    -1,0,-1,
                            0,1,1,    0,-1,1,    0,1,-1,    0,-1,-1,
                            1,1,0,    0,-1,1,    -1,1,0,    0,-1,-1]);
    p_gradient = gl.createTexture();
    gl.bindTexture(gl.TEXTURE_2D, p_gradient);
    gl.texParameteri(gl.TEXTURE_2D, gl.TEXTURE_WRAP_S, gl.REPEAT);
    gl.texParameteri(gl.TEXTURE_2D, gl.TEXTURE_WRAP_T, gl.REPEAT);
    gl.texParameteri(gl.TEXTURE_2D, gl.TEXTURE_MIN_FILTER, gl.LINEAR);
    gl.texParameteri(gl.TEXTURE_2D, gl.TEXTURE_MAG_FILTER, gl.LINEAR);
    gl.texImage2D(gl.TEXTURE_2D, 0, gl.RGB, 16, 1, 0, gl.RGB, gl.UNSIGNED_BYTE, g);

}

var cubeVertexPositionBuffer;
var cubeVertexTextureCoordBuffer;
var cubeVertexIndexBuffer;

function initBuffers() {
    cubeVertexPositionBuffer = gl.createBuffer();
    gl.bindBuffer(gl.ARRAY_BUFFER, cubeVertexPositionBuffer);
    vertices = [
        // Back face
        -1.0, -1.0, -1.0,
        -1.0,  1.0, -1.0,
         1.0,  1.0, -1.0,
         1.0, -1.0, -1.0,
    ];
    gl.bufferData(gl.ARRAY_BUFFER, new Float32Array(vertices), gl.STATIC_DRAW);
    cubeVertexPositionBuffer.itemSize = 3;
    cubeVertexPositionBuffer.numItems = 4;

    cubeVertexTextureCoordBuffer = gl.createBuffer();
    gl.bindBuffer(gl.ARRAY_BUFFER, cubeVertexTextureCoordBuffer);
    var textureCoords = [
      // Back face
      1.0, 0.0,
      1.0, 1.0,
      0.0, 1.0,
      0.0, 0.0,
    ];
    gl.bufferData(gl.ARRAY_BUFFER, new Float32Array(textureCoords), gl.STATIC_DRAW);
    cubeVertexTextureCoordBuffer.itemSize = 2;
    cubeVertexTextureCoordBuffer.numItems = 4;

    cubeVertexIndexBuffer = gl.createBuffer();
    gl.bindBuffer(gl.ELEMENT_ARRAY_BUFFER, cubeVertexIndexBuffer);
    var cubeVertexIndices = [
        0, 1, 2,      0, 2, 3,    // Back face
    ];
    gl.bufferData(gl.ELEMENT_ARRAY_BUFFER, new Uint16Array(cubeVertexIndices), gl.STATIC_DRAW);
    cubeVertexIndexBuffer.itemSize = 1;
    cubeVertexIndexBuffer.numItems = 6;
}

var darkness = 0.0;

function drawScene() {
    gl.viewport(0, 0, gl.viewportWidth, gl.viewportHeight);
    gl.clear(gl.COLOR_BUFFER_BIT | gl.DEPTH_BUFFER_BIT);

    gl.bindBuffer(gl.ARRAY_BUFFER, cubeVertexPositionBuffer);
    gl.vertexAttribPointer(shaderProgram.vertexPositionAttribute, cubeVertexPositionBuffer.itemSize, gl.FLOAT, false, 0, 0);

    gl.bindBuffer(gl.ARRAY_BUFFER, cubeVertexTextureCoordBuffer);
    gl.vertexAttribPointer(shaderProgram.textureCoordAttribute, cubeVertexTextureCoordBuffer.itemSize, gl.FLOAT, false, 0, 0);

    gl.activeTexture(gl.TEXTURE0);
    gl.bindTexture(gl.TEXTURE_2D, p_perm);
    gl.uniform1i(shaderProgram.perm_sampler, 0);
    gl.activeTexture(gl.TEXTURE1);
    gl.bindTexture(gl.TEXTURE_2D, p_gradient);
    gl.uniform1i(shaderProgram.grad_sampler, 1);

    gl.uniform1f(shaderProgram.time, TICK);
    gl.uniform1f(shaderProgram.darkness, darkness);

    gl.bindBuffer(gl.ELEMENT_ARRAY_BUFFER, cubeVertexIndexBuffer);
    gl.drawElements(gl.TRIANGLES, cubeVertexIndexBuffer.numItems, gl.UNSIGNED_SHORT, 0);
}

var TICK = 0.0;
var lastTime = 0;
darkness = 1.0;

function animate() {
    var timeNow = new Date().getTime();
    if (lastTime != 0) {
        var elapsed = timeNow - lastTime;
    }
    lastTime = timeNow;
    TICK += 0.02;
    /*darkness += 0.02;
    if (darkness > 1.0) {
        darkness = 1.0;
    }*/
}


function tick() {
    requestAnimFrame(tick);
    drawScene();
    animate();
}

function maximize(){
    var canvas = document.getElementById("c");

    var width = document.width === undefined ? $(document).width() : document.width;
    var height = document.height === undefined ? $(document).height() : document.height;
    canvas.height = height;
    canvas.width = width;
}

function webGLStart() {        
    var canvas = document.getElementById("c");

    maximize();
    initGL(canvas);
    initShaders();
    initBuffers();
    initTexture();

    gl.clearColor(0.5, 0.5, 0.5, 1.0);
    gl.enable(gl.DEPTH_TEST);

    tick();
}

$(document).ready(function(){
    webGLStart();
    $("#c").mousemove(function(e){
        mouseX = e.pageX / $(window).width(); 
    });
    $(window).resize(function(){
        var canvas = document.getElementById("c");
        maximize();
        try {
            gl = canvas.getContext("experimental-webgl");
            gl.viewportWidth = canvas.width;
            gl.viewportHeight = canvas.height;
        } catch (e) {
            console.log(e);
        }
    });
});
!
999px

Console