Revealjs line highlighting cannot meet WCAG contrast #14959

In revealjs, code-line-numbers highlights lines by fading the other lines to opacity: 0.4 (line-highlight.css#L1-L7). The design comes from the reveal.js highlight plugin, which uses the same value.

At 0.4, the faded lines fail WCAG 1.4.3 Contrast (Minimum). With the default highlight theme on a white background, faded tokens are 1.7:1 to 2.6:1. The requirement is 4.5:1.

No other opacity value is a good fix. The token colors of the theme are close to 4.5:1 at full opacity, so they have almost no room to fade:

At 0.94, the highlighted line looks the same as the other lines. The feature then has no function.

The faded text is not exempt. WCAG gives an exemption only for "incidental" text, and the faded lines are content that the audience reads for context. Sometimes an earlier step shows all the lines at full opacity (code-line-numbers="|2|4-5"). This does not help a person in the audience, because that person cannot go back to the earlier step.

Suggestions

1. Documentation: In the Line Highlighting section, tell authors that the fade does not meet WCAG contrast. Tell authors who need accessible slides not to use code-line-numbers for this purpose.
2. Investigate other methods: Show emphasis on the highlighted lines and keep the other lines at full contrast. Some possible methods are a background color, a left border, or a bold font weight. A method that uses only color must also meet 1.4.11 Non-text Contrast and 1.4.1 Use of Color. This can be a new default or an option.

Repro

---
format: revealjs
---

## Python

```{.python code-line-numbers="2|4-5"}
import numpy as np  # a comment
x = np.linspace(0, 10, 100)
def f(x, k=2):
    """Docstring here."""
    return x ** k + 3.5
```

Quarto 1.11.4, default revealjs theme. The contrast values come from the computed styles in Chrome.

AI tools helped with this investigation. The investigation used a local clone of quarto-cli (see "Using AI tools to investigate" in CONTRIBUTING.md).