Click To Show Running WebGL

Updated data and calls to vertexAttribPointer(). The highlighted lines have the relevant changes.

 var squareVertexBuffer;

    function initBuffers() {
      //Create Square Position Buffer
        squareVertexBuffer = gl.createBuffer();
        gl.bindBuffer(gl.ARRAY_BUFFER, squareVertexBuffer);
        var vertices = [
             1.0,  1.0,  0.0, 1.0, 1.0, //x, y, z, u, v
            -1.0,  1.0,  0.0, 0.0, 1.0,
             1.0, -1.0,  0.0, 1.0, 0.0,
            -1.0, -1.0,  0.0, 0.0, 0.0
        ];
        gl.bufferData(
                     gl.ARRAY_BUFFER, 
                     new Float32Array(vertices), 
                     gl.STATIC_DRAW
                     );
        squareVertexBuffer.stride = 5*Float32Array.BYTES_PER_ELEMENT;
        squareVertexBuffer.positionElementCount = 3;
        squareVertexBuffer.positionOffset = 0;
        squareVertexBuffer.uvElementCount = 2;
        squareVertexBuffer.uvOffset = 3*Float32Array.BYTES_PER_ELEMENT;
        squareVertexBuffer.numItems = 4;

    }

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

        pMatrix = identity();
        mvMatrix = identity();

        pMatrix = ortho(-1, 1, -1, 1, 0.01, 10);
        mvMatrix = makeTranslation(0.0, 0.0, -0.01);

        gl.bindBuffer(gl.ARRAY_BUFFER, squareVertexBuffer);
        gl.vertexAttribPointer(
                              shaderProgram.vertexPositionAttribute,
                              squareVertexBuffer.positionElementCount, 
                              gl.FLOAT, 
                              false, 
                              squareVertexBuffer.stride, 
                              squareVertexBuffer.positionOffset
                              );
        gl.vertexAttribPointer(
                              shaderProgram.vertexUVAttribute, 
                              squareVertexBuffer.uvElementCount, 
                              gl.FLOAT, 
                              false, 
                              squareVertexBuffer.stride, 
                              squareVertexBuffer.uvOffset 
                              );
        setUniforms();
        gl.drawArrays(
                      gl.TRIANGLE_STRIP, 
                      0, 
                      squareVertexBuffer.numItems
                      );
    }

I extended the Full Screen Quad example to handle mouse location with a little visual feedback when you click and drag.

Click To Show Running WebGL

    precision mediump float;
    uniform vec2 uViewportSize;
    uniform vec2 uMouseLocation;
    uniform float uMouseRadius;
    varying vec2 vUV;

    void main(void) {
      vec4 orig_color = vec4(vUV.x, vUV.y, 1.0, 1.0);
      vec4 mouse_color = vec4(1, 0, 0, 1.0);

      float dist = length(
                          vec2(gl_FragCoord) - 
                          vec2(uMouseLocation.x - uMouseRadius,
                               uMouseLocation.y + uMouseRadius
                              )
                         );
      float t =  clamp(dist/uMouseRadius, 0.0, 1.0);

      gl_FragColor = mix(mouse_color, orig_color, t);
    }

   ...
    function initShaders() {
       ...
        shaderProgram.MouseLocationUniform=gl.getUniformLocation(shaderProgram,
                                                             "uMouseLocation");
        shaderProgram.MouseRadiusUniform=gl.getUniformLocation(shaderProgram,
                                                             "uMouseRadius");
    }


   
    function setUniforms() {
        ...
        gl.uniform2f(
                    shaderProgram.MouseLocationUniform, 
                    mouse[0], 
                    mouse[1]
                    );
        gl.uniform1f(
                    shaderProgram.MouseRadiusUniform, 
                    mouseRadius 
                    );

    }



    

    var mouse = [250,250];
    var mouseRadius = 10;
    var currentAngle = [0,0]; // [x-axis, y-axis] degrees

    function webGLStart() {
        ...
        initEventHandlers(canvas, mouse, currentAngle);

        ...
        var tick = function() {
          drawScene();
          requestAnimationFrame(tick,canvas);
        };
        tick();
    }

    function initEventHandlers(canvas, mousePosition, currentAngle)
    {
      var lastX = -1;
      var lastY = -1;
      var dragging = false;

      canvas.onmousedown = function(ev) {  //Mouse is pressed
        var x = ev.clientX;
        var y = ev.clientY;

        var rect = ev.target.getBoundingClientRect();
        if(rect.left <= x 
            &amp;&amp; x <= rect.right 
            &amp;&amp; rect.top <= y 
            &amp;&amp; y <= rect.bottom
          ) {
          lastX = x; 
          lastY = y;
          mousePosition[0] = x;
          mousePosition[1] = canvas.height - y;
          dragging = true;

        }
      };

      canvas.onmouseup = function(ev){ //Mouse is released
        dragging = false;
      }

      canvas.onmousemove = function(ev) { //Mouse is moved
        var x = ev.clientX;
        var y = ev.clientY;
        if(dragging) {
          //put some kind of dragging logic in here
          //Here is a roation example
          var factor = 100/canvas.height;
          var dx = factor * (x - lastX);
          var dy = factor * (y - lastY);
          //Limit x-axis roation angle to -90 to 90 degrees
          currentAngle[0] = Math.max(Math.min(currentAngle[0] + dy, 90), -90);
          currentAngle[1] = currentAngle[1] + dx;

          mousePosition[0] = x;
          mousePosition[1] = canvas.height - y;

        }
        lastX = x;
        lastY = y;

      }
    }
    webGLStart();

Click To Show Running WebGL

Fragment Shader:

precision mediump float;
    uniform vec2 uViewportSize;
    varying vec2 vUV;

    void main(void) {
      gl_FragColor = vec4(vUV.x, vUV.y, 1.0, 1.0);

      /*
      //gl_FragCoord is a way of getting the pixel location

        gl_FragColor = vec4(
                            gl_FragCoord.x/uViewportSize.x, 
                            gl_FragCoord.y/uViewportSize.y, 
                            1.0, 
                            1.0
                           );
     */
    }

Vertex Shader:

 //Data from bufferData
    attribute vec3 aVertexPosition;
    attribute vec2 aVertexUV;

    uniform mat4 uMVMatrix;
    uniform mat4 uPMatrix;
    uniform vec2 uViewportSize;

    //These will be passed to the fragment shader and will be interpreted per
    //pixel
    varying vec2 vUV;

    void main(void) {
        gl_Position = uPMatrix * uMVMatrix * vec4(aVertexPosition, 1.0);
        //The line below will ignore any translation of perspective matrix
        //gl_Position =  vec4(aVertexPosition, 1.0);
        vUV = aVertexUV;
    }

Program Source:

    var gl;
    function initGL(canvas) {
        try {
            //gl = canvas.getContext("webgl");
            gl = canvas.getContext("experimental-webgl");
            gl.viewportWidth = canvas.width;
            gl.viewportHeight = canvas.height;
        } catch (e) {
        }
        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, "shader-fs");
        var vertexShader = getShader(gl, "shader-vs");

        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.vertexUVAttribute = gl.getAttribLocation(shaderProgram, "aVertexUV");
        gl.enableVertexAttribArray(shaderProgram.vertexUVAttribute);

        shaderProgram.pMatrixUniform = gl.getUniformLocation(shaderProgram, "uPMatrix");
        shaderProgram.mvMatrixUniform = gl.getUniformLocation(shaderProgram, "uMVMatrix");
        shaderProgram.ViewportSizeUniform = gl.getUniformLocation(shaderProgram, "uViewportSize");
    }


    var mvMatrix;
    var pMatrix;

    function setUniforms() {
        gl.uniformMatrix4fv(shaderProgram.pMatrixUniform, false, pMatrix);
        gl.uniformMatrix4fv(shaderProgram.mvMatrixUniform, false, mvMatrix);
        gl.uniform2f(
                    shaderProgram.ViewportSizeUniform, 
                    gl.viewportWidth, 
                    gl.viewportHeight
                    );
    }



    var squareVertexPositionBuffer;
    var squareVertexUVBuffer;

    function initBuffers() {
      //Create Square Position Buffer
        squareVertexPositionBuffer = gl.createBuffer();
        gl.bindBuffer(gl.ARRAY_BUFFER, squareVertexPositionBuffer);
        var vertices = [
             1.0,  1.0,  0.0,
            -1.0,  1.0,  0.0,
             1.0, -1.0,  0.0,
            -1.0, -1.0,  0.0
        ];
        gl.bufferData(gl.ARRAY_BUFFER, new Float32Array(vertices), gl.STATIC_DRAW);
        squareVertexPositionBuffer.itemSize = 3;
        squareVertexPositionBuffer.numItems = 4;

      //Create Square UV Buffer
        squareVertexUVBuffer = gl.createBuffer();
        gl.bindBuffer(gl.ARRAY_BUFFER, squareVertexUVBuffer);
        var uvs = [
             1.0, 1.0,
             0.0, 1.0,
             1.0, 0.0,
             0.0, 0.0
        ];
        
        gl.bufferData(gl.ARRAY_BUFFER, new Float32Array(uvs), gl.STATIC_DRAW);
        squareVertexUVBuffer.itemSize = 2;
        squareVertexUVBuffer.numItems = 4;
    }


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


        pMatrix = identity();
        mvMatrix = identity();

        //pMatrix = ortho(-1, 1, -1, 1, 0.01, 10);
        //pMatrix = makePerspective(45 * Math.PI/180.0, gl.viewportWidth / gl.viewportHeight, 0.1, 100.0);
        //mvMatrix = makeTranslation(0.0, 0.0, -0.01);

        gl.bindBuffer(gl.ARRAY_BUFFER, squareVertexPositionBuffer);
        gl.vertexAttribPointer(shaderProgram.vertexPositionAttribute, squareVertexPositionBuffer.itemSize, gl.FLOAT, false, 0, 0);
        gl.bindBuffer(gl.ARRAY_BUFFER, squareVertexUVBuffer);
        gl.vertexAttribPointer(shaderProgram.vertexUVAttribute, squareVertexUVBuffer.itemSize, gl.FLOAT, false, 0, 0);
        setUniforms();
        gl.drawArrays(gl.TRIANGLE_STRIP, 0, squareVertexPositionBuffer.numItems);
    }



    function webGLStart() {
        var canvas = document.getElementById("webgl-canvas");
        initGL(canvas);
        initShaders();
        initBuffers();

        gl.clearColor(0.0, 0.0, 0.0, 1.0);
        gl.enable(gl.DEPTH_TEST);

        drawScene();
    }
    
    webGLStart();

It seems that on OS X 10.7+ if you go to the Preferences->Advanced and check the box “Show Develop menu in menu bar.” You can then go to the Develop menu and choose “Enable WebGL.”

Advanced Preferences