As previously stated, I’ve recently been working on a selector engine. Unfortunately, as every browser behaves slightly differently, I’ve had to resort to constructing compatibility tables. Seeing as they can be useful in other applications, I thought I’d post it here:

Element.querySelectorAll

A browser-native selector engine that allows you to select element nodes based on a CSS selector (for example: div p a).

var a = document.getElementById('a');

a.querySelectorAll('div.post > p a[class]');

Element.firstElementChild / Element.lastElementChild / Element.previousElementSibling / Element.nextElementSibling

Selects the first/last element child or the previous/next element sibling. Saves on extrenuous code like:

var previous = document.getElementById('example');
while((previous = previous.previousSibling) !== null && previous.nodeType !== 1) { }

var a = document.getElementById('a');

a.firstElementChild;
a.lastElementChild;
a.previousElementSibling;
a.nextElementSibling;

Element.children

Similar to the childNodes method, but without text nodes and comments.

var a = document.getElementById('a');

a.children;

Element.compareDocumentPosition

Allows you to compare one node’s position to another. Returns a bitmask.

var a = document.getElementById('a');
var b = document.getElementById('b');

a.compareDocumentPosition(b);

Element.contains

Returns a boolean whether or not an element is contained within another.

var a = document.getElementById('a');
var b = document.getElementById('b');

a.contains(b);