* {
    box-sizing: border-box;
    margin: 0;
}
body {
    padding: 10px;
}
fieldset {
    border: 1px solid;
    display: flex;
    flex-direction: column;
    padding-top: 0;
    width: 100%;
}
form > fieldset > *:nth-child(even) {
    margin-top: 20px;
}
fieldset fieldset {
    border-color: #DDDDDD;
}
fieldset fieldset label,
fieldset fieldset .box-wrapper {
    margin-top: 10px;
}
input[type=text],
input[type=number] {
    width: 100%;
}
textarea {
    height: 150px;
    padding: 3px;
    width: 100%;
}
.box-wrapper {
    line-height: 30px;
    margin-top: 5px;
}
.box-wrapper > label {
    display: block;
    margin: -30px 0 0 25px;
}
.controls {
    text-align: center;
}

* {
    box-sizing: border-box;
    margin: 0;
}
body {
    min-width: 400px;
    padding: 10px;
}
fieldset {
    border: 1px solid;
    display: flex;
    flex-direction: column;
    padding-top: 0;
    width: 100%;
}
form > fieldset > *:nth-child(even) {
    margin-top: 20px;
}
fieldset fieldset {
    border-color: #DDDDDD;
}
fieldset fieldset label,
fieldset fieldset .box-wrapper {
    margin-top: 10px;
}
input[type=text],
input[type=number] {
    width: 100%;
}
textarea {
    height: 150px;
    padding: 3px;
    width: 100%;
}
.box-wrapper {
    line-height: 30px;
    margin-top: 5px;
}
.box-wrapper > label {
    display: block;
    margin: -30px 0 0 25px;
}
.controls {
    text-align: center;
}

Fix min-width to give browser action popup a usable width
Fix min-width to give browser action popup a usable width
