Make a button fill the full width of container element?

Add the “box-sizing: border-box” property to all elements. The width and height properties include the padding and border, but not the margin. This will ensure your measurements across elements are correct and take into account the padding. display: block and width: 100% is the correct way to go full width but you need to remove the left/right margin on the button as it pushes it outside the containing element.

* {
    box-sizing: border-box
} 

.container {
    background-color: #ddd;
    padding: 10px;
    margin: 0 auto;
    max-width: 500px;
}

.button {
    background-color: #bbb;
    display: block;
    margin: 10px 0;
    padding: 10px;
    width: 100%;
}

for more information on the box-sizing property, read the MDN docs.