5.5 button | click

<!DOCTYPE html>
<meta charset="utf-8">
<html>
<head>

<link href="https://fonts.googleapis.com/css2?family=Montserrat+Alternates:ital,wght@0,100;0,200;0,300;0,400;0,500;0,600;0,700;0,800;0,900;1,100;1,200;1,300;1,400;1,500;1,600;1,700;1,800;1,900&family=Montserrat:ital,wght@0,100;0,200;0,300;0,400;0,500;0,600;0,700;0,800;0,900;1,100;1,200;1,300;1,400;1,500;1,600;1,700;1,800;1,900&family=Roboto:ital,wght@0,100;0,300;1,100;1,300&display=swap" rel="stylesheet">

<script src="https://d3js.org/d3.v4.min.js"></script>

<p id="h1">BUTTON | CLICK</p>
<div id="link">by
<a href="https://slides.com/sandravizmad">SANDRA</a></div>

<button id="start" style="position: absolute; margin-left:55px; margin-top: 120px;">START</button>
<button id="reset" style="position: absolute; margin-left:130px; margin-top: 120px;">RESET</button>

<style>

/*Defining text stylings*/

#h1 {
  font-size:30px;
  margin:30px 0px 0px 20px;
  color:#f5fa91;
  font-family:'Montserrat Alternates', sans-serif;
  font-weight:300;
}

#link {
	font-family:'Montserrat Alternates', sans-serif;
  font-weight:200;
  font-size:10px;
	margin:5px 0px 0px 22px;
  color:white;
}

a:link, a:visited, a:active {
	text-decoration: none;
  color:white;
  border-bottom:1.5px dotted white;
}

body {
  background-color:#011227;
}

/*Defining button stylings*/

#start, #reset {
  font-family:'Montserrat Alternates', sans-serif;
  font-size:14px;
  color:#8ff798;
  border:0;
  text-align:center;
  background-color:#011227;
}

#start:hover, #reset:hover {
  color:white;
  background-color:#011227;
  border:0;
}

</style>
</head>

//Margin conventions
var m = {top:100, right:50, bottom:50, left:40}
    w = 800 - m.left - m.right,
    h = 600 - m.top - m.bottom;

//Container
var svg = d3.select("body")
    .append("svg")
    .attr("width", w + m.left + m.right)
    .attr("height", h + m.top + m.bottom)
    .append("g")
		.attr("transform", `translate(${m.left}, ${m.top})`);

//Draw the rect
var rect=svg.append("rect")
		        .attr("x", 0)
		        .attr("y", 50)
		        .attr("width", 350)
		        .attr("height", 350)
            .attr("fill", "#85f6fa");

//Click start button
d3.select("#start")
  .on("click", function() {

rect
    .attr("width", 0)

});

//Click reset button

d3.select("#reset")
  .on("click", function() {

rect
    .attr("x", 0)
    .attr("y", 50)
    .attr("width", 300)
    .attr("height", 300)
    .attr("fill", "#85f6fa");

});

</script>
</body>
</html>

https://d3js.org/d3.v4.min.js