Use native form validation!
Chrome about e-commerce sites, some great stuff there. - might be useful for 1stdibs.com.
When to www
The only technical reason is that if you usehttp://example.com, requests to subdomains (egstatic.example.com) will be sent the cookies sent on the main domain. This is seen as a disadvantage because you might want to use a subdomain for serving static content, and the cookies add more overhead.
This only happens when you set the cookie for .example.com (notice the dot prefix) domain name. If you set a cookie for example.com domain, www.example.com will not get/send that cookie.
However that’s not entirely true. It only makes a difference if you have enough cookies that they don’t fit in one packet (~1kb). And even then, it only really matters if you’re Google or Facebook.
Also with HTTP/2, using multiple subdomains (aka “domain sharding”) is actually detrimental - it’s faster to have everything on 1-2 domains max.
Infrastructure
React repository infrastructure
About why it's worth using proper html and css
UX
Use save data header or the browser API to save your users bandwidth.
navigator.hardwareConcurrency