字母x的下邊緣(線)就是我們的基線。

這段大概解釋了line box中基線的位置

CSS 2.1 does not define the position of the line box's baseline (i.e., the position of the strut, see below).

然后這段話說的字體之間會按基線對齊，但有也提到?jīng)]有字體的時候，行內(nèi)框的基線是strut，也就解釋了inline box基線的位置

User agent must align the glyphs in a non-replaced inline box to each other by their relevant baselines. Then, for each glyph, determine the A and D. Note that glyphs in a single element may come from different fonts and thus need not all have the same A and D. If the inline box contains no glyphs at all, it is considered to contain a strut (an invisible glyph of zero width) with the A and D of the element's first available font.

有了以上兩條，可以以知道對于行內(nèi)級元素他的parent box不論是inline box還是line box對齊都是參照strut（0寬字符）