How important are web standards? Have you ever tried the W3C Markup Validation Service on sites other than yours or your competitors? Check out the Google Homepage: 20 issues. Bing: 57. Amazon: 79. (Stats updated September 1, 2017)
Maybe it's just me, but for some of the biggest heavyweights on the web, they don't seem to be overly concerned with their code validating.
Do you validate your HTML and CSS?
Meanwhile across the web, standardistas flip out every time the W3C comes out with a new or updated spec; it becomes a race to see who can build a site that will validate first so we can say "I am using valid HTML5". It doesn't feel like this is a very good thing, though. As web developers, we shouldn't be focussing on standards compliance just so we can feel like we're a part of an elitist club.
Your clients (and their visitors) don't care about web standards
Does standards compliant code make for a more accessible site? It can, but it isn't guaranteed. Are you going to attract a significant number of people to a site just because it's code matches up to what the World Wide Web Consortium has decreed as valid? Unlikely. After all, invalid code doesn't seem to be hurting Amazon's traffic much, does it? If we strive to produce the best code we can, we will by definition be aiming to produce valid code, but we should also temper our work with sound business decisions as well.
The truth is, unless the visitors to a particular site are web developers, they probably don't care—nor will they ever know—whether your site validates. In fact, the average visitor to a site doesn't even know what the term "valid code" means. (I do see an obvious exception with sites where web developers are the primary visitors.)
What should be done?
Every web developer should be striving to produce valid code. It definitely has its advantages over the long haul, and when considering the SEO advantages it provides, it only makes sense. Beyond that, however, if you have minor validation errors that aren't causing visitors problems, let it go and move on.
Photo credit schoschie