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Revealjs line highlighting cannot meet WCAG contrast

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评估

难度
4/5
预计耗时
3-5 天
新手友好度
48/100
Issue 类型
缺陷
描述清晰度
基本清楚
活跃度
活跃
技术栈
css, javascript
领域
accessibility, documentation, frontend

调研方向

Start with src/resources/formats/revealjs/plugins/line-highlight/line-highlight.css and reproduce the contrast measurements using the provided Quarto revealjs example. Review the Line Highlighting section of the revealjs documentation, then determine whether the completed change should document the limitation, provide an accessible emphasis method, or both; done means the documentation and any chosen presentation behavior address the stated WCAG concerns.

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描述

accessibility revealjs

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:

Token Color Contrast at 1.0 Contrast at 0.4 Minimum opacity for 4.5:1
attribute (.at) #657422 5.15 1.74 0.94
import (.im) #00769e 5.14 1.81 0.93
comment (.co) #5e5e5e 6.48 1.85 0.85
plain text #111111 18.88 2.62 0.58

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).

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  3. Fork 仓库，在一个分支上完成修改。
  4. 提交 Pull Request，并在描述里引用这个 Issue 编号。

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