css-extras

Each example now includes a comment showing what the CSS output would be or describing the behavior. The outputs help users understand: Exact values for mathematical functions (e.g., --negate(1em) → -1em) Color transformations with color space notation (e.g., oklab(), oklch(), rgb()) Responsive behaviors for fluid functions (e.g., viewport-based scaling) Descriptive outputs for complex functions (e.g., multi-layered shadows, elastic easing)

The demo site at https://sindresorhus.com/css-extras/example.html looks great in light mode but it has bad contrast in dark mode:

This PR adds a function that calculates the proportion of two values (`calc( / )calc()transform: scale(tan(atan2(, )));viewBoxtan(atan2(, ))--proportion()` function in this PR does.