CSS button will not align at center of div - html

Question

I have this website with a showcase div, within the div I have a "Get Started" button. I am able to vertically align the button with the margin attribute. However I do not want to horizontally align the button with margin as this will lead to some trouble for me in the future, I've tried align: center; and align="center" but the button sticks to the left side of the showcase. How can I unstick this button and horizontally align it without having to use a margin attribute?
html,
body,
header {
position: absolute;
left: 0;
top: 0;
width: 100%;
margin: 0;
margin-top: 0;
margin: auto;
padding: 0;
}
h1 {
display: inline-block;
font-family: Trebuchet MS;
font-size: 75px;
letter-spacing: 3px;
margin: 10px 0px 0px 20px;
}
h2 {
display: inline-block;
flex-direction: row;
margin: 0px 0px 0px 50px;
font-family: Georgia;
}
.highlight {
color: #45d845;
}
.heading {
background-color: #d84545;
border-bottom: 5px solid black;
padding-top: 20px;
padding-bottom: 20px;
}
#comet {
font-size: 65px;
margin-left: 20px;
}
.showcase {
background: url('background1.jpeg');
border-bottom: 5px solid black;
height: 1000px;
width: 100%;
background-size: 100% 100%;
background-repeat: no-repeat;
color: #cccccc;
}
.showcase h2 {
color: #fff;
margin-top: 170px;
font-size: 60px;
font-family: Verdana;
text-align: center;
text-shadow: 1px 3px #000;
}
#start {
align: center;
margin-top: 130px;
background-color: transparent;
color: #fff;
padding: 20px 30px 20px 30px;
text-align: center;
font-family: Helvetica;
font-weight: bold;
border-radius: 10px;
-webkit-transition-duration: 0.4s;
transition-duration: 0.4s;
}
#start:hover {
cursor: pointer;
background-color: #fff;
color: #000;
}
<header>
<div class="heading">
<h1>ThumbTac <span id="comet">☄</span> </h1>
</div>
</header>
<div class="showcase">
<h2>He moonlight difficult engrossed an it sportsmen. Interested has all devonshire difficulty jay assistance joy. Unaffected at ye </h2>
<button id="start" align="center">Get Started</button>
</div>

Answer 1

You can wrap your button in a div with the style text-align: center; to horizontally center your button. Example:
html, body, header{
position:absolute;
left:0;
top:0;
width: 100%;
margin: 0;
margin-top: 0;
margin: auto;
padding: 0;
}
/*Heading*/
h1{
display: inline-block;
font-family: Trebuchet MS;
font-size: 75px;
letter-spacing: 3px;
margin: 10px 0px 0px 20px;
}
h2{
display: inline-block;
flex-direction: row;
margin: 0px 0px 0px 50px;
font-family: Georgia;
}
.highlight{
color: #45d845;
}
.heading{
background-color: #d84545;
border-bottom: 5px solid black;
padding-top: 20px;
padding-bottom: 20px;
}
#comet{
font-size: 65px;
margin-left: 20px;
}
/*Showcase*/
.showcase{
background: url('background1.jpeg');
border-bottom: 5px solid black;
height: 1000px;
width: 100%;
background-size: 100% 100%;
background-repeat: no-repeat;
color: #cccccc;
}
.showcase h2{
color: #fff;
margin-top: 170px;
font-size: 60px;
font-family: Verndana;
text-align: center;
text-shadow: 1px 3px #000;
}
#start{
align: center;
margin-top: 130px;
background-color: transparent;
color: #fff;
padding: 20px 30px 20px 30px;
text-align: center;
font-family: Helvetica;
font-weight: bold;
border-radius: 10px;
-webkit-transition-duration: 0.4s; /* Safari */
transition-duration: 0.4s;
}
#start:hover{
cursor: pointer;
background-color: #fff;
color: #000;
}
<header>
<div class="heading">
<h1>ThumbTac <span id="comet">☄</span> </h1>
</div>
</header>
<div class="showcase">
<h2>He moonlight difficult engrossed an it sportsmen.
Interested has all devonshire difficulty jay
assistance joy. Unaffected at ye </h2>
<div style="text-align: center;">
<button id="start">Get Started</button>
</div>
</div>

Answer 2

You can consider using a flexbox. Documentation in the CSS source.
/* Global Sets */
html,
body,
header {
position: absolute;
left: 0;
top: 0;
width: 100%;
margin: 0;
margin-top: 0;
margin: auto;
padding: 0;
}
/*Heading*/
h1 {
display: inline-block;
font-family: Trebuchet MS;
font-size: 75px;
letter-spacing: 3px;
margin: 10px 0px 0px 20px;
}
h2 {
display: inline-block;
flex-direction: row;
margin: 0px 0px 0px 50px;
font-family: Georgia;
}
.highlight {
color: #45d845;
}
.heading {
background-color: #d84545;
border-bottom: 5px solid black;
padding-top: 20px;
padding-bottom: 20px;
}
#comet {
font-size: 65px;
margin-left: 20px;
}
/*Showcase*/
.showcase {
background: url('background1.jpeg');
border-bottom: 5px solid black;
height: 1000px;
width: 100%;
background-size: 100% 100%;
background-repeat: no-repeat;
color: #cccccc;
display: flex; /* Added */
flex-direction: column; /* Added */
align-items: center; /* Added, horizonal alignment */
}
.showcase h2 {
color: #fff;
margin-top: 170px;
font-size: 60px;
font-family: Verndana;
text-shadow: 1px 3px #000;
text-align: center;
}
#start {
/* align: center; Not valid CSS */
margin-top: 130px;
background-color: transparent;
color: #fff;
padding: 20px 30px 20px 30px;
/* text-align: center; No longer required */
font-family: Helvetica;
font-weight: bold;
border-radius: 10px;
-webkit-transition-duration: 0.4s;
/* Safari */
transition-duration: 0.4s;
}
#start:hover {
cursor: pointer;
background-color: #fff;
color: #000;
}
<header>
<div class="heading">
<h1>ThumbTac <span id="comet">☄</span> </h1>
</div>
</header>
<div class="showcase">
<h2>He moonlight difficult engrossed an it sportsmen. Interested has all devonshire difficulty jay assistance joy. Unaffected at ye </h2>
<button id="start" align="center">Get Started</button>
</div>