Third div automatically floating

Question

I can not understand how css works, and it's annoying me. I was trying to do some basic side by side two divs and one div below them. At first I've learned that I had to give float:left for both side by side divs. For curiosity I did'nt gave float:left for the second side by side div, and I came across this layout:

_{(source: imge.to)}

Then I gave float:left for the second side by side div, and I came across this layout:

_{(source: imge.to)}

Question: I didn't gave float:left for third div but it didn't act like the first screen shot. Why?

css code:

* {
    margin: 0;
    padding: 0;
    box-sizing: border-box;
}

.container {
    width: 1000px;
    margin-left: auto;
    margin-right: auto;
}

.blog-posts {
    width: 50%;
    background-color: #0000ff;
    float: left;
}

.other-posts {
    width: 25%;
    background-color: #00ff00;
    float: left;
}

.author-text {
    background-color: #ffff00;
}

html code:

    <div class="container">
        <div class="blog-posts">dend endje denjde akdlsd gsjgıdg sadsujrg spsadnajd asdnsajdd</div>
        <div class="other-posts">extra dummy text</div>
        <div class="author-text">author text</div>
    </div>

Answer 1

Wrap the items you want side by side in another wrapper, then apply flexbox to that wrapper:

.my-flex-wrap {
  display: flex;
}

Then remove all the floats. Done.

* {
  margin: 0;
  padding: 0;
  box-sizing: border-box;
}

.container {
  width: 1000px;
  margin-left: auto;
  margin-right: auto;
}

.my-flex-wrap {
  display: flex;
}

.blog-posts {
  width: 50%;
  background-color: #0000ff;
}

.other-posts {
  width: 25%;
  background-color: #00ff00;
}

.author-text {
  background-color: #ffff00;
}

<div class="container">
  <div class="my-flex-wrap">
    <div class="blog-posts">dend endje denjde akdlsd gsjg?dg sadsujrg spsadnajd asdnsajdd</div>
    <div class="other-posts">extra dummy text</div>
  </div>
  <div class="author-text">author text</div>
</div>