If you had to hire a frontend developer and had just one question, what would you ask?

I recommend the 3-second frontend developer test:

Do you validate [your HTML and CSS code]?

A truthful answer gives you more than any other question would do.

Why is that?

The answer tells you several important things:

1. Foundational knowledge: Through validating and the correcting of validation issues, the developer is sure to have and to maintain foundational knowledge of development with HTML and CSS. A developer not validating may or may not have this knowledge.

2. Base quality: Assuming a validation routine, validation ensures base quality of the output. A developer who doesn’t validate, on the other hand, is guaranteed to deliver inferior quality. Guaranteed, because even with perfect HTML and CSS knowledge, they will make mistakes. No one writes perfect code, all the time, and a senior frontend developer is not permitted an excuse from checking their code on issues.

3. Professional conduct: Frontend development is open to everyone. To this day, there is nothing tangible that differentiates a professional frontend developer from a wannabe or amateur developer. (I’m close to recording how my houseplants write HTML and CSS less invalid than that of the most popular sites.) Nothing differentiates professional and amateur developers—except for a habit of validation. A developer who doesn’t validate and who doesn’t ship valid code is not a professional developer.

4. Aspiration: The practice of validation suggests aspiration; the developer is not content with using HTML and CSS incorrectly, with not using them professionally. A developer not validating their HTML and CSS does not demonstrate any more aspiration than a driver revving their car’s engine in the central business district. That may appear competitive, but they’re a far cry from a professional race driver.

Validation is important. Validation is the thing that makes frontend developers professional frontend developers. Validation is therefore a key criterion for hiring. By applying it, you set a well-defined bar, and you help the field of frontend development level up as a profession and a craft. Be a frontend developer who validates and ships valid HTML and CSS; hire frontend developers who validate and ship valid HTML and CSS.

PS.
The few exceptions do prove the rule. They may consist in pending updates to validators, but can also relate to false positives. They do not consist in a developer or an organization putting themselves above the rule—especially not when their code is shared. As the impact is larger, even higher standards apply here. The time of unconditionally accepted sloppiness in professional frontend development must end.

PPS.
If you like another perspective at the benefits of validation, Why Validate Your Pages? tries to be super-comprehensive. If you like a list of (web-based) validators, UITest.com’s “conformance” section features the main ones, covering other languages as well (disclosure: I run UITest.com).