TA貢獻1963條經(jīng)驗 獲得超6個贊
W3C標準的算法里,float可能參與父級高度計算，但position:absolute;始終不參與父級計算。就是這樣設計的，沒轍：W3CCSS2.110.6.3規(guī)定了未創(chuàng)建BFC的非替換塊級元素的高度計算規(guī)則：Thissectionalsoappliestoblock-levelnon-replacedelementsinnormalflowwhen'overflow'doesnotcomputeto'visible'buthasbeenpropagatedtotheviewport.If'margin-top',or'margin-bottom'are'auto',theirusedvalueis0.If'height'is'auto',theheightdependsonwhethertheelementhasanyblock-levelchildrenandwhetherithaspaddingorborders:Theelement'sheightisthedistancefromitstopcontentedgetothefirstapplicableofthefollowing:thebottomedgeofthelastlinebox,iftheboxestablishesainlineformattingcontextwithoneormorelinesthebottomedgeofthebottom(possiblycollapsed)marginofitslastin-flowchild,ifthechild'sbottommargindoesnotcollapsewiththeelement'sbottommarginthebottomborderedgeofthelastin-flowchildwhosetopmargindoesn'tcollapsewiththeelement'sbottommarginzero,otherwise如上述分點，分項1定義了line-box撐高容器，分項2和3定義了常規(guī)流塊級元素撐高容器（margin折疊與否的兩種情況），否則為0。W3CCSS2.110.6.7規(guī)定了創(chuàng)建了BFC的元素（blockformattingcontextroots）的高度計算規(guī)則：Ifitonlyhasinline-levelchildren,theheightisthedistancebetweenthetopofthetopmostlineboxandthebottomofthebottommostlinebox.Ifithasblock-levelchildren,theheightisthedistancebetweenthetopmargin-edgeofthetopmostblock-levelchildboxandthebottommargin-edgeofthebottommostblock-levelchildbox.Absolutelypositionedchildrenareignored,andrelativelypositionedboxesareconsideredwithouttheiroffset.Notethatthechildboxmaybeananonymousblockbox.Inaddition,iftheelementhasanyfloatingdescendantswhosebottommarginedgeisbelowtheelement'sbottomcontentedge,thentheheightisincreasedtoincludethoseedges.Onlyfloatsthatparticipateinthisblockformattingcontextaretakenintoaccount,e.g.,floatsinsideabsolutelypositioneddescendantsorotherfloatsarenot.這里規(guī)則大致跟上方一樣，不同點在于margin不再折疊和float高度參與計算。總之呢，在“撐高父容器圈”里面沒有絕對定位元素的地位。
TA貢獻1828條經(jīng)驗 獲得超4個贊
position為absolute的元素是相對于離他最近的position不為static的父級元素定位的，它不會占據(jù)父元素的空間，所以也不會撐起父級元素，這點和設置float的元素不一樣，唯一的辦法是給父級元素設置高度。
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