Extra space in responsive css using float:left; - css

Question

For the life of me, I cannot figure out why this code can't work. I am trying to set up a personal website and before I put my content in place, I want to have all of the areas setup and have it be responsive. I want a 3x3 grid of boxes where I can display my work (div id = container), but every time I introduce the ninth div block specifically (p9), the arrangement breaks for seemingly no reason. Here's the code for the desktop layout:
body{
background-color:#FFB51E;
width:100%;
height:1000px;
position:absolute;
}
/* unvisited link */
a:link {
text-decoration:none;
background-color: #2A56C4;
color:#fff;
padding:15px;
border-radius:26px;
}
/* visited link */
a:visited {
color: fff;
}
/* mouse over link */
a:hover {
background-color:#FF581E;
}
/* selected link */
a:active {
color:#FF581E;
background-color:fff;
}
#header{
width:80%;
height:160px;
margin: 0 auto;
position:relative;
display:block;
}
.left{
color:#fff;
text-align: left;
margin-top:25px;
margin-bottom:15px;
font-size:36px;
position:relative;
float:left;
width:310px;
display:block;
}
.right{
text-align:right;
width:300px;
float:right;
padding-top:5px;
margin-bottom:15px;
font-size:24px;
position:relative;
float:right;
z-index:2;
}
.works{
text-align:center;
position:relative;
float:left;
left:30%;
font-size:25px;
width:100px;
display:inline-block;
}
.about{
text-align:center;
position:relative;
float:right;
right:30%;
font-size:25px;
width:100px;
display:inline-block;
}
.border{
background-color:none;
width:100%;
height:85px;
margin:0 auto;
border:none;
border-bottom: 6px solid #000;
float:none;
position:relative;
}
/*body stuff*/
#container{
position:static;
display:block;
margin:0 auto;
font-size:0px;
margin-top: -10px;
width:80%;
height:550px;
}
.p1{
background-color: aliceblue;
color:000;
font-size:12px;
width:230px;
z-index: 1;
float:left;
margin: 0px;
padding:0px;
}
.p2{
background-color: red;
width:230px;
z-index: 1;
float:left;
padding:0px;
}
.p3{
background-color: blue;
width:230px;
z-index: 1;
float:left;
padding:0px;
margin:0px;
}
.p4{
background-color: purple;
width:230px;
z-index: 1;
float:left;
margin-top: 20px;
padding:0px;
}
.p5{
background-color: green;
width:230px;
z-index: 1;
float:left;
margin-top: 20px;
padding:0px;
}
.p6{
background-color: brown;
width:230px;
z-index: 1;
float:left;
padding:0px;
margin-top: 20px;
}
.p7{
background-color: purple;
width:230px;
z-index: 1;
float:left;
margin-top: 20px;
padding:0px;
}
.p8{
background-color: green;
width:230px;
z-index: 1;
float:left;
margin-top: 20px;
padding:0px;
}
.p9{
background-color: green;
width:230px;
z-index: 1;
float:left;
margin-top: 20px;
padding:0px;
}
I'm about five minutes from drop kicking my laptop out the window, so any kind of help would be greatly appreciated! Let me know if you need code for the html as well.

Answer 1

Something to get you started. I don't have the HTML you use so I focused on the container.
I defined it as a flexbox which makes it responsive. Each item has a width of 33% and a height of 30px (for demo purpose).
/*body stuff*/
#container {
display: flex;
flex-wrap: wrap;
margin: 0 auto;
margin-top: -10px;
width: 80%;
}
[class^="p"] {
width: 33%;
height: 30px;
}
.p1 {
background-color: aliceblue;
}
.p2 {
background-color: red;
}
.p3 {
background-color: blue;
}
.p4 {
background-color: purple;
}
.p5 {
background-color: green;
}
.p6 {
background-color: brown;
}
.p7 {
background-color: yellow;
}
.p8 {
background-color: red;
}
.p9 {
background-color: green;
}
<div id="container">
<div class="p1"></div>
<div class="p2"></div>
<div class="p3"></div>
<div class="p4"></div>
<div class="p5"></div>
<div class="p6"></div>
<div class="p7"></div>
<div class="p8"></div>
<div class="p9"></div>
</div>

Answer 2

First: I just added this CSS rule at the bottom (to overwrite the other rules) and now it works as desired:
#container > div {
width: 230px;
margin-top: 20px;
}
https://jsfiddle.net/bhzw7o60/1/
Second: For elements that have common parameters (like your floated elements which all have the same width, size and margin-top) you should use a * common* class for all of them and additional seperate classes for the individual elements which only contain the differing properties. My above rule does this for width and margin-top. You could also add the height to it, and only define the background-color in the individual classes. BTW: z-index does nothing in this case, you can delete that from all the rules.