W3C validation offers the litmus test of everything the browser renders. If it doesn't pass those filters, it simply isn't valid markup! I keep checking for errors during development and only zero errors spell absolute perfection.
I tend to think with a mobile-first frame of mind since it's the shortest route to conversion, but also enjoy working with explicitly larger screens and resolutions for POS / POI design or arty high-end desktop experiences.
All resources are finite. That goes for memory, processing power, and everything else partaining to computers. Nobody likes latency so apps and sites must be scalable in nature. That can be tricky in the open-source landscape, but there are many tried-and-true libraries that have made their way into high-end production code.
Who likes vendor lock-in? Who wants to paint their clients into a particular corner? If there is one complaint I have heard from clients is their inability to take back the reigns from a full-service commercial agency. Therefore all code I write privately is portable to the max.
I've worked with Zend Framework 1 + 2 when they were all the rage, and I kind of enjoyed the simplicity of the Table model and MVC structure. It allowed me to build scalable apps with a a decent ACL. It's the opposite of lightweight though, and I've scaled back my usage of Zend Framework lately.
ProcessWire offers an open source CMS solution capable of handling almost every contingency you can throw at it. Everthing you miss and everything you like about every other CMS - ProcessWire seems to have it out of the box and throws in a delicious API. I love the non-opinionated approach to templating and even configuring it is simple bliss.
AngularJS lets you extend HTML vocabulary for your application. It takes an unopinionated MVC approach so it's a strong choice for both single-page apps as dynamic websites.
In the end there are tools for everything (especially on a Linux/Windows machine) but it's great to have access to the professional Adobe design products. They have been a part of my toolset since my education so they form a natural environment for me.
Despite being designed for advanced 3D, the extra tools found in Cinema 4D Studio are still designed to be user-friendly and intuitive. I generally use Cinema 4D to export models to OBJ format so they can be loaded into BabylonJS, and rendering concept materials for actual in-store displays.
My code editor of choice.