Improve abstract in HTML template.

* Add localized title "abstract", unless `abstract-title` variable
  is set.
* Add `abstract-title` div to abstract CSS.
* Move abstract CSS out of CSL conditional.
* Ensure that abstract is aligned left but indented on all sides.
* Use smaller font for abstract.

Improves #7588.

data/templates/default.html4

@@ -48,6 +48,7 @@ $if(date)$
 $endif$
 $if(abstract)$
 <div class="abstract">
+<div class="abstract-title">$abstract-title$</div>
 $abstract$
 </div>
 $endif$

data/templates/default.html5

@@ -51,6 +51,7 @@ $if(date)$
 $endif$
 $if(abstract)$
 <div class="abstract">
+<div class="abstract-title">$abstract-title$</div>
 $abstract$
 </div>
 $endif$

data/templates/styles.html

@@ -73,6 +73,19 @@ blockquote {
   border-left: 2px solid #e6e6e6;
   color: #606060;
 }
+$if(abstract)$
+div.abstract {
+  margin: 2em 2em 2em 2em;
+  text-align: left;
+  font-size: 85%;
+}
+div.abstract-title {
+  font-weight: bold;
+  text-align: center;
+  padding: 0;
+  margin-bottom: 0.5em;
+}
+$endif$
 code {
   font-family: $if(monofont)$$monofont$$else$Menlo, Monaco, 'Lucida Console', Consolas, monospace$endif$;
 $if(monobackgroundcolor)$
@@ -161,12 +174,6 @@ div.csl-entry {
 $if(csl-entry-spacing)$
   margin-bottom: $csl-entry-spacing$;
 $endif$
-$if(abstract)$
-div.abstract{
-  margin: auto;
-  width: 90%;
-}
-$endif$
 }
 .hanging div.csl-entry {
   margin-left:2em;

src/Text/Pandoc/Writers/HTML.hs

@@ -43,6 +43,7 @@ import Text.DocLayout (render, literal, Doc)
 import Text.Blaze.Internal (MarkupM (Empty), customLeaf, customParent)
 import Text.DocTemplates (FromContext (lookupContext), Context (..))
 import Text.Blaze.Html hiding (contents)
+import Text.Pandoc.Translations (Term(Abstract))
 import Text.Pandoc.Definition
 import Text.Pandoc.Highlighting (formatHtmlBlock, formatHtmlInline, highlight,
                                  styleToCss)
@@ -65,7 +66,8 @@ import System.FilePath (takeBaseName)
 import Text.Blaze.Html.Renderer.Text (renderHtml)
 import qualified Text.Blaze.XHtml1.Transitional as H
 import qualified Text.Blaze.XHtml1.Transitional.Attributes as A
-import Text.Pandoc.Class.PandocMonad (PandocMonad, report)
+import Text.Pandoc.Class.PandocMonad (PandocMonad, report,
+                                      translateTerm)
 import Text.Pandoc.Class.PandocPure (runPure)
 import Text.Pandoc.Error
 import Text.Pandoc.Logging
@@ -267,6 +269,7 @@ pandocToHtml opts (Pandoc meta blocks) = do
   let descriptionMeta = literal $ escapeStringForXML $
                           lookupMetaString "description" meta
   slideVariant <- gets stSlideVariant
+  abstractTitle <- translateTerm Abstract
   let sects = adjustNumbers opts $
               makeSections (writerNumberSections opts) Nothing $
               if slideVariant == NoSlides
@@ -355,6 +358,7 @@ pandocToHtml opts (Pandoc meta blocks) = do
                   (if stMath st
                       then defField "math" math
                       else id) .
+                  defField "abstract-title" abstractTitle .
                   (case writerHTMLMathMethod opts of
                         MathJax u -> defField "mathjax" True .
                                      defField "mathjaxurl"