As mention by @AethanFoot #472 (comment) I accidentally broke the parsing behavior of Margins.

Existing behavior before my change:

• no values: gives a warning, assume 10px on all side
• 1 value: assume that value on all side (match CSS)
• 2 values: assume the first value for top and right and the second value for bottom and left
  ⚠️ This is contradicting CSS where 2 values means: first value for top and bottom and second value for right and left
• 3 values: assume the first value for top, second value for right, third value for bottom and left
  ⚠️ This is contradicting CSS where 3 values means: first value for top, second value for right and left, third value for bottom
• 4 values: top, right, bottom, left (match CSS)
• more than 4 values: gives a warning, assume 4 values

What I implemented:

• no values: assume 0 on all side, no warning ❌
• 1 value: assume that value on all side (match CSS) ✔️
• 2 to 3 values: assign value to top, right, bottom and left, use 0 if one is missing ❌
• 4 values: top, right, bottom, left (match CSS) ✔️
• more than 4 values: does not parse (fatal) ❌

❌ means different from original behavior
✔️ means identical to original behavior

This PR restore the original behavior but I think we should modify it to comply with CSS so it will feel more intuitive. And I think 0 or more than 4 values should be parsing errors (fatal) so it will shows in leftwm-check.

Let me know your opinion.