Why is the white box appearing?

Question

The first box the loop outputs is a white box with a black outline. I don't understand where it's coming from... https://editor.p5js.org/matranson/present/6fNelJM8_

function setup() {
    colorMode(HSB,360,100,100);
    createCanvas(400, 400);
    var boxh = height / 10;
    var boxw = width;
    for(var i = 0; i < 10; i++) {
    var h = lerp(64, 22, i / 9);
    var s = lerp(86, 90, i / 9);
    var l = lerp(96, 56, i / 9);
    rect(0, i * boxh, boxw, boxh);
    fill(h,s,l);   
    stroke(0,0,100);
    }
    
}

function draw() {
}

<script src="https://cdnjs.cloudflare.com/ajax/libs/p5.js/0.8.0/p5.js"></script>

Answer 1

stroke() sets the color used to draw lines and borders. fill() sets the color used to fill shapes. rect() draws a rectangle.
The stroke and the fill color has to be set before the rectangle is drawn:

fill(h,s,l);   
stroke(0,0,100); 
rect(0, i * boxh, boxw, boxh); 

function setup() {
    colorMode(HSB,360,100,100);
    createCanvas(400, 400);
    var boxh = height / 10;
    var boxw = width;
    for(var i = 0; i < 10; i++) {
      var h = lerp(64, 22, i / 9);
      var s = lerp(86, 90, i / 9);
      var l = lerp(96, 56, i / 9);
    
      fill(h,s,l);   
      stroke(0,0,100); 
      rect(0, i * boxh, boxw, boxh);
    }
}

<script src="https://cdnjs.cloudflare.com/ajax/libs/p5.js/0.8.0/p5.js"></script>