HTML writer: use flex property for column widths

Fixes: #8232

src/Text/Pandoc/Writers/HTML.hs

@@ -876,7 +876,7 @@ blockToHtmlInner opts (Div attr@(ident, classes, kvs') bs) = do
   let isCslBibEntry = "csl-entry" `elem` classes
   let kvs = [(k,v) | (k,v) <- kvs'
                    , k /= "width" || "column" `notElem` classes] ++
-            [("style", "width:" <> w <> ";") | "column" `elem` classes
+            [("style", "flex:" <> w <> ";")  | "column" `elem` classes
                                              , ("width", w) <- kvs'] ++
             [("role", "doc-bibliography") | isCslBibBody && html5] ++
             [("role", "doc-biblioentry") | isCslBibEntry && html5]

test/command/1710.md

@@ -19,17 +19,17 @@ ok
 <section id="slide-one" class="slide level1">
 <h1>Slide one</h1>
 <div class="columns">
-<div class="column" style="width:40%;">
+<div class="column" style="flex:40%;">
 <ul>
 <li>a</li>
 <li>b</li>
 </ul>
-</div><div class="column" style="width:40%;">
+</div><div class="column" style="flex:40%;">
 <ul>
 <li>c</li>
 <li>d</li>
 </ul>
-</div><div class="column" style="width:10%;">
+</div><div class="column" style="flex:10%;">
 <p>ok</p>
 </div>
 </div>