Depending on the vessel (ship):
Stern (back), port (left), bow (forward), starboard (right). These directions are based on the vessel "cap".

Depending on the skiers:
"skier's Left" is based on a skier facing down the mountain.

Depending on the reader:

• inline is how a sentence is read
• block is the perpendicular of inline
• start is where the flow... starts
• end is where the flow ... ends

I find it still hard to explain and I got it with examples. Read more on mdn

block/inline and start/end can be mixed up to target a specific location. From our Latin reader perspective, it applies as follows: https://i0.wp.com/css-tricks.com/wp-content/uploads/2022/03/block-dir.png?w=720&ssl=1

It's user-centric because its language direction is respective. [...] We can embrace diversity, complexity, and unpredictability and rely on the browser engines to properly lay it out.

Why all of these? Because right, left, top, and bottom can be interpreted differently between the explainer's and the public points of view.