Because getElementsByTagName()
returns a NodeList, not a single element. Treat it as array:
document.getElementsByTagName("body")[0].style.display = "block";
Or even simpler in case of body
:
document.body.style.display = "block";
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…