Table of Contents
Most sudokus can be solved using the very basic techniques as lone singles and hidden singles. The difficult part is finding hidden singles. Setting pencil marks helps here.
The more advance techniques are unfortunately rarely required.
https://www.linurs.org/sudoku/index.html has just the basic techniques implemented and therefore might not be able to solve all sudokus. The sodokus not being able to solve are therefore the interesting ones that are worth to study.