HTML img and video tags should always have their width and heights included.

Other HTML elements will automatically change their size depending on their contents and the size of their screen/containers.

It is possible to set the width and/or the heights of elements to meet specific needs.

width

This will move the main page sections into a central, 800px wide part of the screen:

 header, nav, main, footer {
    width: 800px;
    margin: auto;
}

height

Some pages may have single line headings, some double line headings. Fixing the height of the header will give a consistent appearance across pages:

 header  {
    height: 50px
}