top not work to get the yellow box down?

This question already has an answer here:

Why does this CSS margin-top style not work? 11 answers

---

This happens due to margin collapsing - so a border , padding to the parent element or inline content (any inline element) will switch off margin collapsing.

See demo below:

.largebox {
  width: 800px;
  height: 350px;
  background-color: #00f;
  margin-left: 10px;
  border: 1px solid; /*ADDED THIS*/

}
.box1 {
  width: 250px;
  height: 300px;
  background-color: #ff0;
  margin-left: 50px;
  margin-top: 25px;
}

<div class="largebox">
  <div class="box1"></div>

</div>

---

Use display:inline-block; in box1

.largebox {
    width: 800px;
    height: 350px;
    background-color: #00f;
    //padding-left: 50px;
    margin-left: 10px;
    //border: 2px solid black;
}

.box1 {
    width: 250px;
    height: 300px;
    background-color: #ff0;
    //display: inline;
    //float: left;
    //margin-right: 0px;
    margin-left: 50px;
    margin-top: 25px;
    display:inline-block;
 }
 
 

<div class="largebox">
        <div class="box1"></div>

    </div>

---

You can try using position:absolute; in .box1 like this:

.box1{
    position:absolute;
}

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