Hayato left a rather flattering review comment to my pull request for integrating shadow tree event dispatch into the DOM Standard. It made me reflect upon all the effort that came before us with regards to adding components to DOM and HTML. It has been a nearly two-decade journey to get to a point where all browsers are willing to implement, and then ship. It is not quite a glacial pace, but you can see why folks say that about standards.
What I think was the first proposal was simply titled HTML Components, better known as HTC, a technology by the Microsoft Internet Explorer team. Then in 2000, published in early 2001, came XBL, a technology developed at Netscape by Dave Hyatt (now at Apple). In some form that variant of XBL still lives on in Firefox today, although at this point it is considered technical debt.
In 2004 we got sXBL and in 2006 XBL 2.0, the latter largely driven by Ian Hickson with design input from Dave Hyatt. sXBL had various design disputes that could not be resolved among the participants. Selectors versus XPath was a big one. Though even with XBL 2.0 the lesson that namespaces are an unnecessary evil for rather tightly coupled languages was not yet learned. A late half-hearted revision of XBL 2.0 did drop most of the XML aspects, but by that time interest had waned.
There was another multi-year gap and then from 2011 onwards the Google Chrome team put effort into a different, more API-y approach towards HTML components. This was rather contentious initially, but after recent compromises with regards to encapsulation, constructors for custom elements, and moving from selectors to an even more simplistic model (basically strings), this seems to be the winning formula. A lot of it is now part of the DOM Standard and we also started updating the HTML Standard to account for shadow trees, e.g., making sure script elements execute.
Hopefully implementations follow soon and then widespread usage to cement it for a long time to come.