One way to hold header: head + -er, the thing at the head of something. Think of the word head as meaning the front end or the top end, not only the part of a body. Whatever sits at that end, and whatever gets there first, can be a header. On a page it is the strip across the top. In a file or a message it is the part the machine reads before anything else. And when a whole person goes head end first — into a ball, into a swimming pool — that movement gets the same name.
Wiktionary's entry gives header as head + -er, the -er ending coming through Old English -ere. That is exactly the shape of the modern picture: the noun names a thing belonging to the head, meaning the top or front end.
Wiktionary 把 header 記成 head 加上 -er，-er 這個字尾經由古英語 -ere 進入英文。這跟上面那張畫面正好吻合：這個名詞指的就是「屬於頭那一端的東西」，而頭在這裡指最上面或最前面。
Wiktionary (kaikki extract, source_ref kaikki:3f6f69a190036ae4:910372989), etymology_text: From head + -er
Two branches
排在最前面的那一塊
這一群都是「放在最前端、先給人看的資訊」。差別只在被放在什麼東西的前面：一頁紙、一段文字、一封信，還是一串資料。
noun · B1
The strip at the top of a page, and the information printed in it — often a page number, a date, a title, or a name that repeats in the same place on every page of a document.
頁首
頭先出去的那一下
這一群不是「放在前面的資訊」，而是「頭這一端先動」。跟上面共用的是同一個 head，但方向從空間位置換成了身體動作，所以另外給它一張畫面。
noun · B1
In soccer, a move in which a player hits the ball with their head, usually to pass it to someone else or to send it into the goal.
頭球