Build the design system in code, and make it accessible by default.
Why this one
Accessibility is what I was originally brought in for, and the lesson from every site that bolts it on later is that it never really gets done. So I built it in at the component level: if the button, the card, the menu, and the product media are accessible by default, then every page assembled from them inherits that, and a new template can't quietly ship an inaccessible pattern. The same library that kept the brand visually consistent across dozens of templates is what kept it accessible across all of them. Doing both in one system, rather than as two separate efforts, is the whole point.
The trade-off
Building accessibility into every component up front is slower than shipping first and auditing later, and some of the work is invisible until someone navigates with a keyboard or a screen reader. It's worth it because accessibility done at the end is a pile of one-off fixes that regress the moment someone adds a page. Done in the system, it holds.