How to restrict width div element into table element

I have css file

table {
  border: 1px solid;
  width: 400px;
}

And i have html file

<link rel="stylesheet" href="mytablediv.css">
<table><tr><td>table</td></tr></table>
<table><tr><td><div style="width:1000px;">div</div></td></tr></table>

I can’t change html file but i can change css file.

How to restrict width div element into table element and make it 400px?

I tried div{ max-width:100% !important;} and it dont’t work.
Width of table with div is 1000px but i need 400 px.

>Solution :

Its just a valid solution, because you cant change the html!

table div {  width: 100% !important; }