Why does my float elements not working properly at 40% + 60% width settings?

Question

I have the following CSS and HTML set:

html, body{

  margin: 0;

}



header{

  border: 1px solid green;

  background-color: green;

  height: 50px;

}



nav{

  border: 1px solid red;

  background-color: red;

  height: 50px;

}



aside{

  border: 1px solid yellow;

  background-color: yellow;

  height: 50px;

  width: 40%;

  float: left;

}



section{

  border: 1px solid blue;

  background-color: blue;

  height: 50px;

  width: 60%;

  float: right;

}

<body>

  <header>

    

  </header>

  <nav>

    

  </nav>

  <aside>

    

  </aside>

  <section>

    

  </section>

</body>

But I'm confused as to how my

<aside>

and

<section>

are not side by side even after setting them to float properties. I set

<aside>

to

width: 40%

and

<section>

to

width: 60%

. Shouldn't they add up to

100%

and fill the entire webpage horizontally across?

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Answer 1

This is happening because of 4px of extra space is being covered by borders in aside and section boxes. Just provide box-sizing: border-box to both;

html, body{
  margin: 0;
}

header{
  border: 1px solid green;
  background-color: green;
  height: 50px;
}

nav{
  border: 1px solid red;
  background-color: red;
  height: 50px;
}

aside{
  border: 1px solid yellow;
  background-color: yellow;
  height: 50px;
  width: 40%;
  float: left;
  box-sizing: border-box;
}

section{
  border: 1px solid blue;
  background-color: blue;
  height: 50px;
  width: 60%;
  float: right;
  box-sizing: border-box;
}

<body>
  <header>
    
  </header>
  <nav>
    
  </nav>
  <aside>
    
  </aside>
  <section>
    
  </section>
</body>

Why does my float elements not working properly at 40% + 60% width settings?

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