element nodes share methods with document, such as
getElementById
getElementByClassName
getElementByTagName
and properties such as
childNodes
And you also have properties like
nextSibling
previousSibling
parentNode
So while document.getElementsByTagName('tr') would retrieve all tds in a document, if el is a table el.getElementsByTagName('tr') retrieves all tds of that table and if el2 is a tr, el2.getElementsByTagName('tr') retrieves all of that row's tds.