If you were to pick one web development blog post that played a huge part in changing or influencing how you build on the web—which blog post would it be?

@zachleat Not really a "blog post", but the HTML Design Principles has been pretty foundational in how I think about both making websites and making web standards:
https://www.w3.org/TR/html-design-principles/

In particular, the Priority of Constituencies ("In case of conflict, consider users over authors over implementors over specifiers over theoretical purity.") is a guiding mandate in standards design.

HTML Design Principles