REM 字体大小未调整到任意阈值以下

Question

在 Safari 12.0.2 和 Mac Mojave 10.14.2 上的 Chrome 71.0.3578.98 中，当使用 rem 单位设置 font-size 时，实际大小为低于 9px.

看这个例子：

https://codepen.io/stephenjwatkins/pen/OrbGxL

我的浏览器的字体大小设置为默认 (16px)，最小字体大小设置为 6px:

将 text-size-adjust 设置为 none 不会影响问题。 Firefox 正确呈现大小。

我发现唯一可以解决此问题的方法是将 font-size: 0; 设置为父元素。例如，如果将 font-size: 0; 添加到 .container，则会呈现正确的字体大小。

有谁知道为什么它不接受低于特定阈值的 rem 大小？

Answer 1

Chrome 及其 Blink 渲染引擎似乎有一些不明显的字体缩放规则。我不知道有任何官方综合文档，所以让我们去源代码吧。

（请注意，我一般不是 Chromium 内部结构方面的专家，也不是 Blink 渲染器方面的专家。我只是一直在追踪源代码并推测所提出问题的最可能答案.)

在我看来，引擎调用 the FontBuilder class during a redraw. This class has various dispatch methods that pass the DOM, zoom, and other relevant factors into what appears to be the crucial method: FontSize :: getComputedSizeFromSpecifiedSize。在这种方法中，我们看到了一些有趣的评论来解决您提出的问题：

1.为什么将 font-size: 0; 设置为父元素可以修复它？

  // Text with a 0px font size should not be visible and therefore needs to be
  // exempt from minimum font size rules. Acid3 relies on this for pixel-perfect
  // rendering. This is also compatible with other browsers that have minimum
  // font size settings (e.g. Firefox).

2。为什么它不尊重低于某个阈值的 rem 大小？

  // We support two types of minimum font size. The first is a hard override
  // that applies to all fonts. This is "minSize." The second type of minimum
  // font size is a "smart minimum" that is applied only when the Web page can't
  // know what size it really asked for, e.g., when it uses logical sizes like
  // "small" or expresses the font-size as a percentage of the user's default
  // font setting.

  // With the smart minimum, we never want to get smaller than the minimum font
  // size to keep fonts readable. However we always allow the page to set an
  // explicit pixel size that is smaller, since sites will mis-render otherwise
  // (e.g., http://www.gamespot.com with a 9px minimum).

3。出于好奇，当给定相对单位（例如 x-small）时，这些最小值是多少？

// Strict mode table matches MacIE and Mozilla's settings exactly.
static const int strictFontSizeTable[fontSizeTableMax - fontSizeTableMin +
                                     1][totalKeywords] = {
    {9, 9, 9, 9, 11, 14, 18, 27},    {9, 9, 9, 10, 12, 15, 20, 30},
    {9, 9, 10, 11, 13, 17, 22, 33},  {9, 9, 10, 12, 14, 18, 24, 36},
    {9, 10, 12, 13, 16, 20, 26, 39},  // fixed font default (13)
    {9, 10, 12, 14, 17, 21, 28, 42}, {9, 10, 13, 15, 18, 23, 30, 45},
    {9, 10, 13, 16, 18, 24, 32, 48}  // proportional font default (16)
};
// HTML       1      2      3      4      5      6      7
// CSS  xxs   xs     s      m      l     xl     xxl
//                          |
//                      user pref

有趣的是，FontBuilder class 调度到 TextAutosizer :: computeAutosizedFontSize 以缩放字体大小。此方法使用硬编码值和可变比例因子：

  // Somewhat arbitrary "pleasant" font size.
  const float pleasantSize = 16;
  // Multiply fonts that the page author has specified to be larger than
  // pleasantSize by less and less, until huge fonts are not increased at all.
  // For specifiedSize between 0 and pleasantSize we directly apply the
  // multiplier; hence for specifiedSize == pleasantSize, computedSize will be
  // multiplier * pleasantSize. For greater specifiedSizes we want to
  // gradually fade out the multiplier, so for every 1px increase in
  // specifiedSize beyond pleasantSize we will only increase computedSize
  // by gradientAfterPleasantSize px until we meet the
  // computedSize = specifiedSize line, after which we stay on that line (so
  // then every 1px increase in specifiedSize increases computedSize by 1px).
  const float gradientAfterPleasantSize = 0.5;

从这些事实中，我们看到有大量的硬编码像素值，9 和 16 通常散布在相关代码中。这些硬代码、将字体缩小到极限的若干规则的存在以及使用字体大小覆盖的能力似乎都符合观察结果并表明它的行为符合预期——即使不一定直观。

此外，我发现 Chrome bug #319623 中发布的最新评论与您的报告非常相似。

Possibly related: when using relative units on the html tag, rem-based values defined elsewhere will have a lower bound of 9px.

See CodePen: http://codepen.io/larrybotha/pen/wKYYXE

Workaround: absolute unit on html, em unit on body. rems everywhere else.

观察该错误以进一步开发可能是谨慎的，尽管可能不会屏住呼吸。最后一次更新是在 2015 年。

REM 字体大小未调整到任意阈值以下

REM font size not adjusting below arbitrary threshold

css

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