craft
Implementing the primitives rather than importing them
Writing the underlying maths and algorithms rather than reaching for a library, and therefore knowing what they do, what they cost, and where they break.
Not a rule against libraries. A component implemented once can be debugged, bounded and reasoned about under load; an imported one is a black box until it misbehaves.
Evidence3
A real-time renderer built from the maths up
I wrote the linear algebra myself rather than pulling in glm: 1,259 lines covering Vec2/3/4, Mat2/3/4, transpose, the projection and model matrices, and a 4x4 inverse.
This site, as a career database
I implemented relative luminance and the contrast ratio from the WCAG definition and tested them against the specification's own worked values, rather than taking a colour library's word for it.
Microscopy image processing and 3D reconstruction
Filters compose through a factory that returns chainable functions over raw FFT convolution, and the volumetric path runs marching cubes and then smooths, decimates and exports the mesh. I built that path rather than calling a one-shot conversion.