How can I have a div like this? - html

Question

I want to have a welcome page like this:
But instead I get this:
* {
box-sizing: border-box;
margin: 0;
padding: 0;
}
html,
body {
background-color: #000000;
margin: 0;
height: 90%;
width: 100%;
align-items: center;
}
#container1 {
height: 100%;
vertical-align: middle;
display: table-cell;
background-color: yellow;
display: flex;
align-items: center;
}
#left {
height: 500px;
color: white;
background-color: blue;
font-size: 20px;
float: left;
width: 100%;
}
#right {
height: 500px;
color: white;
background-color: red;
font-size: 20px;
float: left;
width: 100%;
}
<main id="container1" class="container my-6">
<div class="">
<div id="left" class="col-lg-6 my-3">
</div>
</div>
<div class="">
<div id="right" class="col-lg-6 my-3">
</div>
</div>
</main>
I don't know why my container doesn't fully fit the body of the page, and my left and right don't go in the middle and stretch width to each other's end.

Answer 1

You have a bunch of errors in your code. I commented out the CSS you don't need:
No need for float, that's what flex is for.
display: table-cell is being overwritten by display: flex
Use flex to set the properties of your left and right divs.
Remove the containing elements around those.
* {
box-sizing: border-box;
margin: 0;
padding: 0;
}
html,
body {
background-color: #000000;
margin: 0;
height: 90%;
width: 100%;
/* NOT NEEDED: align-items: center;*/
}
#container1 {
width: 100%;
height: 100%;
vertical-align: middle;
/* NOT NEEDED: display: table-cell; */
background-color: yellow;
display: flex;
/* This is probably unneeded. align-items, aligns elements on the cross access - which in this case would be vertically aligned in the center since flex-direction by default, is row */
align-items: center;
}
#left {
height: 500px;
color: white;
background-color: blue;
font-size: 20px;
/* NOT NEEDED float: left; */
/* NOT NEEDED width: 100%; */
flex: 1 1 50%;
}
#right {
height: 500px;
color: white;
background-color: red;
font-size: 20px;
flex: 1 1 50%;
/* NOT NEEDED float: left; */
/* NOT NEEDED width: 100%; */
}
<main id="container1" class="container my-6">
<div id="left" class="col-lg-6 my-3">
</div>
<div id="right" class="col-lg-6 my-3">
</div>
</main>

Answer 2

The problem comes mostly from the divs without classes, that shouldn't be there.
But you're also mixing floats, with flex and tables. Just stick with flex like in this example:
html, body{
width: 100%;
height: 100%;
padding: 0;
margin: 0;
}
.container{
width: 100%;
height: 100%;
display: flex;
}
.left,
.right {
width: 50%;
height: 100%;
}
.left {
background: #215f40;
}
.right {
background: #092414;
}
<div class="container">
<div class="left"></div>
<div class="right"></div>
</div>