Short, practical explanations of the mistakes the checker looks for, written from the vendors' own design documents. Each guide links to its primary sources.
ESP32 strapping pins and the EN circuit
The GPIOs each ESP32 family samples at reset, the GPIO12 flash-voltage trap, and the 10 kΩ + 1 µF EN delay Espressif recommends.
USB-C CC resistors
Why a USB-C sink needs its own 5.1 kΩ on CC1 and on CC2, and why boards with one shared resistor only work with USB-A cables.
I2C pull-up resistors (with calculator)
Rp(min) and Rp(max) from the NXP I2C specification, worked examples and a calculator for your supply, bus capacitance and speed.
Decoupling capacitors
100 nF per power pin, bulk capacitance per rail, analog supplies and placement - with the ST, Espressif and Raspberry Pi recommendations side by side.
Crystal load capacitors (with calculator)
Pick C1 and C2 from the crystal's CL and your stray capacitance instead of defaulting to 22 pF.
STM32 BOOT0, NRST, VCAP and VDDA
The five pins behind most "my new STM32 board does nothing" posts, from ST's hardware getting-started notes.
How to export a KiCad netlist
File > Export > Netlist in KiCad 7 to 10, the kicad-cli command, and fixes for common export problems.