See JavaScript. Following paragraph contains

[A] [B] [C] [D]

(where [B] and [C] are in elements, [A] and [D] in nodes)

Then the contents of different nodes / elements are:

firstChild of paragraph: [A]
firstElementChild of paragraph: [B]
lastChild of paragraph: [D]
lastElementChild of paragraph: [C]
nextElementSibling of B: [C]
nextSibling of C: [D]
previousElementSibling of C: [B]
previousSibling of B: [A]
lastElementChild of parent of B: [C]