Set div to have its siblings width

Answer 1

Not sure if I understood what you are trying to do, but looks like setting a 100% width to the last div should work:

<div style="width:100%;"> 

BTW the style in the first div is not well defined, you should use a colon instead of a equal sign in the properties definition:

<div style="display:inline;">

Answer 2

Floats and tables are so 2000 and late. With today's browsers we can make the two sibling DIVs match each other's width, regardless which is bigger/smaller.

Here's a Flexbox solution fit for 2016:

.wrapper {
  display: inline-block;
}

.parent {
  display: flex;
  flex-direction: column;
}

/* For visualization */
.child {
  border: 1px solid #0EA2E8;
  margin: 2px;
  padding: 1px 5px;
}

<div class="wrapper">
  <div class="parent">
    <div class="child">Child number one</div>
    <div class="child">Child #2</div>
  </div>
</div>