[html] Style input element to fill remaining width of its container

you can try this :

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_x000D_
div#panel {_x000D_
    border:solid;_x000D_
    width:500px;_x000D_
    height:300px;_x000D_
}_x000D_
div#content {_x000D_
 height:90%;_x000D_
 background-color:#1ea8d1; /*light blue*/_x000D_
}_x000D_
div#panel input {_x000D_
 width:100%;_x000D_
 height:10%;_x000D_
 /*make input doesnt overflow inside div*/_x000D_
 -webkit-box-sizing: border-box;_x000D_
       -moz-box-sizing: border-box;_x000D_
            box-sizing: border-box;_x000D_
 /*make input doesnt overflow inside div*/_x000D_
}
_x000D_
<div id="panel">_x000D_
  <div id="content"></div>_x000D_
  <input type="text" placeholder="write here..."/>_x000D_
</div>
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