Input resize 100

https://www.w3schools.com/css/tryit.asp?filename=trycss3_box-sizing_all

<!DOCTYPE html>
<html>
<head>
<style>
body {
margin: 10;
}

* {
box-sizing: border-box;
}

input, textarea {
width: 100%;
}
</style>
</head>
<body>

<form action="/action_page.php">
First name:<br>
<input type="text" name="firstname" value="Mickey"><br>
Last name:<br>
<input type="text" name="lastname" value="Mouse"><br>
Comments:<br>
<textarea name="message" rows="5" cols="30">
</textarea>
<br><br>
<input type="submit" value="Submit">
</form>

<p><strong>Tip:</strong> Try to remove the box-sizing property from the style element and look what happens.
Notice that the width of input, textarea, and submit button will go outside of the screen.</p>

</body>
</html>