I tried these solutions but never got a conclusive result. In the end I used proper semantic markup with a fieldset. It saved having to add any width calculations and any box-sizing.
It also allows you to set the form width as you require and the inputs remain within the padding you need for your edges.
In this example I have put a border on the form and fieldset and an opaque background on the legend and fieldset so you can see how they overlap and sit with each other.
<html>
<head>
<style>
form {
width: 300px;
margin: 0 auto;
border: 1px solid;
}
fieldset {
border: 0;
margin: 0;
padding: 0 20px 10px;
border: 1px solid blue;
background: rgba(0,0,0,.2);
}
legend {
background: rgba(0,0,0,.2);
width: 100%;
margin: 0 -20px;
padding: 2px 20px;
color: $col1;
border: 0;
}
input[type="email"],
input[type="password"],
button {
width: 100%;
margin: 0 0 10px;
padding: 0 10px;
}
input[type="email"],
input[type="password"] {
line-height: 22px;
font-size: 16px;
}
button {
line-height: 26px;
font-size: 20px;
}
</style>
</head>
<body>
<form>
<fieldset>
<legend>Log in</legend>
<p>You may need some content here, a message?</p>
<input type="email" id="email" name="email" placeholder="Email" value=""/>
<input type="password" id="password" name="password" placeholder="password" value=""/>
<button type="submit">Login</button>
</fieldset>
</form>
</body>
</html>