In your CSS, add
position: fixed;
to your header element. It's just that simple, really.
And next time, try to use right click on something you see on website and choose "Inspect element". I think that every modern browser has it now. Very useful function.
与恶龙缠斗过久,自身亦成为恶龙；凝视深渊过久,深渊将回以凝视…
