在 CSS 中，有两种不同类型的字体系列名称：
- 通用字体系列（generic family name） - 拥有相似外观的字体系统组合（比如 "Serif" 或 "Monospace"）
- 特定字体系列（font family name） - 具体的字体系列（比如 "Times" 或 "Courier"）
除了各种特定的字体系列外，CSS 定义了 5 种通用字体系列：
- Serif 字体
- Sans-serif 字体
- Monospace 字体
- Cursive 字体
- Fantasy 字体
font property font-family does not have generic default 是编辑器在提醒我们CSS font-famiy需要配置用来 fallback 的 generic-name。
原文如下（链接https://www.w3.org/TR/CSS21/fonts.html#generic-font-families）：
Style sheet designers are encouraged to offer a generic font family as a last alternative. Generic font family names are keywords and must NOT be quoted.
Font family names must either be given quoted as strings, or unquoted as a sequence of one or more identifiers. This means most punctuation characters and digits at the start of each token must be escaped in unquoted font family names.
For example, the following declarations are invalid:
font-family: Red/Black, sans-serif; font-family: "Lucida" Grande, sans-serif; font-family: Ahem!, sans-serif; font-family: test@foo, sans-serif; font-family: #POUND, sans-serif; font-family: Hawaii 5-0, sans-serif;
If a sequence of identifiers is given as a font family name, the computed value is the name converted to a string by joining all the identifiers in the sequence by single spaces.
To avoid mistakes in escaping, it is recommended to quote font family names that contain white space, digits, or punctuation characters other than hyphens:
body { font-family: "New Century Schoolbook", serif }
<BODY STYLE="font-family: '21st Century', fantasy">
Font family names that happen to be the same as a keyword value ('inherit', 'serif', 'sans-serif', 'monospace', 'fantasy', and 'cursive') must be quoted to prevent confusion with the keywords with the same names. The keywords 'initial' and 'default' are reserved for future use and must also be quoted when used as font names. UAs must not consider these keywords as matching the '<family-name>' type.
Generic font families are a fallback mechanism, a means of preserving some of the style sheet author's intent in the worst case when none of the specified fonts can be selected. For optimum typographic control, particular named fonts should be used in style sheets.
All five generic font families are defined to exist in all CSS implementations (they need not necessarily map to five distinct actual fonts). User agents should provide reasonable default choices for the generic font families, which express the characteristics of each family as well as possible within the limits allowed by the underlying technology.
User agents are encouraged to allow users to select alternative choices for the generic fonts.