Css REM字体大小未调整到任意阈值以下

在Mac Mojave 10.14.2上的Safari 12.0.2和Chrome 71.0.3578.98中，当使用

rem

单位设置

font size

时，实际大小不会低于

9px

请参见此示例：

我的浏览器的字体大小设置为默认值（

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

我发现唯一能解决这个问题的方法是设置

font size:0到父元素。例如，如果添加
字体大小：0
到
.container
，将呈现正确的字体大小

有人知道为什么它不遵守低于某个阈值的
rem
大小吗？
Chrome及其闪烁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的综合文档，所以让我们来看看源代码

（请注意，我不是铬内部构件方面的专家，也不是铬渲染器方面的专家。我只是对源代码进行了跟踪，并推测了所提出问题的最可能答案。）

在我看来，引擎在重画时调用。此类具有各种分派方法，这些方法将DOM、zoom和其他相关因素传递到关键方法中：。在这种方法中，我们看到一些有趣的评论，它们解决了您提出的问题：

1。为什么设置
字体大小：0到父元素修复它？

  // 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).

  // 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).

// 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

2。为什么不承认rem大小低于某个阈值？

  // 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).

  // 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).

// 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

3。好奇的是，当给定相对单位（例如
x-small
）时，这些最小值是多少？

  // 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).

  // 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).

// 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
类调度以缩放字体大小。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，我们可以看到存在大量硬编码像素值，
9
和
16
通常被散布在相关代码中。这些硬代码，几个将字体缩小到一定限度的规则的存在，以及使用字体大小覆盖的能力，似乎都符合观察结果，表明它的行为符合预期——如果不一定是直觉的话


另外，我发现最近发表的评论与你的报告非常相似

可能相关：在html标记上使用相对单位时，其他地方定义的基于rem的值的下限为9px

见代码笔：

解决方法：html上的绝对单位，正文上的em单位。其他地方都有雷姆斯

谨慎的做法是观察该bug的进一步发展，尽管可能不会屏住呼吸。上次更新是在2015年。
当您添加供应商前缀
-webkit text size adjust:none
仍然没有爱？仍然没有爱，供应商前缀为
-webkit text size adjust:none。这是一个已知功能，我将尝试查找重复的问题。我相信这是的重复问题，但由于悬赏，我无法结束投票。这不是的重复问题。浏览器上的最小字体大小设置为
6px
，但它不能使用
rem
单位。例如，如果我明确使用
px
，我可以将字体大小设置为
6px
，但不能低于预期的
6px
。这是一个不同的问题。好的研究和写作。虽然我不能说我完全赞同Blink的逻辑，但基于这个问题和你在这里分享的代码，我同意你的结论。很高兴这有帮助，@StephenWatkins。我希望这一分析至少是在球场上进行的，但我希望有一位Chrome专家打电话进来：这一切对我来说似乎有点不太妥当。解决方法只适用于文本的字体大小。不幸的是，如果你在像divs这样的容器上使用rem来表示宽度/高度，那么你就不能使用。今年早些时候，在chrome73中，出现了与最小字体大小和rem单位相关的问题。因此，Chrome现在允许最小字体大小为零，并且他们也更改了用户界面的设置。但是如果你将你的最小字体大小设置为非零，它可以将你的rem单位放大到很大。UI从0跳到6px，作为可用的最小大小，两者之间没有任何内容。我也希望看到一位Chrome专家进一步详细解释这一点。