瀏覽器裡 JavaScript 的事件觸發順序是先 capture 再 bubble，這個在「Event order」這邊就有一些歷史解釋，IE8 以前只有 capture 模式，到了 IE9+ 才支援，在「Event API: bubbles」這邊也可以看到。
但如果是同一個節點上面的事件觸發順序 (假設同樣是 capture 或是同樣是 bubble)，在「Are event handlers in JavaScript called in order?」這邊有些整理資料。
2000 年的「Document Object Model (DOM) Level 2 Events Specification」這邊提到沒有定義順序：
When the event reaches the target, any event listeners registered on the EventTarget are triggered. Although all EventListeners on the EventTarget are guaranteed to be triggered by any event which is received by that EventTarget, no specification is made as to the order in which they will receive the event with regards to the other EventListeners on the EventTarget.
在早期的 draft「Document Object Model (DOM) Level 3 Events Specification」裡面可以看到：
Next, the implementation must determine the current target's candidate event listeners. This must be the list of all event listeners that have been registered on the current target in their order of registration. [HTML5] defines the ordering of listeners registered through event handler attributes.
但在最新的「UI Events」(要注意這還是 draft，在 2016 年更新的) 則是拿掉了這段。
所以在設計架構時，正常還是得保守的假設沒有保證執行順序...