Why is the layout of this element floating? How to fix?

Question

css

I am currently taking a PHP training course and doing assignments for a training project.
Unfortunately, my knowledge of layout and CSS is very superficial, and I just can’t figure out why my layout is floating? The first option is a training page on pure HTML + CSS, everything is displayed exactly
https://i.imgur.com/tdz4EGK.jpg
And the second option is the code copied from this page, which is loaded as a php template. The only thing changed in the code is the substitution of values, depending on the requested id. That is, I didn’t even touch HTML or CSS. But for some reason (even if I just load pure HTML code as a php template), my layout floats.
https://i.imgur.com/wyjdtrl.jpg
Tell me how to fix this?
Just in case, I will attach a description of the styles "nav" , "nav__list" and "containter" :

.nav {
  background-color: #ffffff;
  border-bottom: 1px solid #e1e6e7;
}
 
.nav__list {
  display: -webkit-flex;
  display: -ms-flexbox;
  display: flex;
  -webkit-justify-content: space-between;
  -ms-flex-pack: justify;
  justify-content: space-between;
  -webkit-flex-wrap: wrap;
  -ms-flex-wrap: wrap;
  flex-wrap: wrap;
  list-style: none;
 
.container {
  width: 1110px;
  margin: 0 auto;
  padding: 0 15px;
}