Close #9450: mathjax: Load MathJax via "defer" strategy

To allow configure MathJax via static JS file, it should be loaded via
"defer" strategy.

CHANGES

@@ -23,6 +23,7 @@ Incompatible changes
 * #9695: The rendering of Javascript domain declarations is implemented
   with more docutils nodes to allow better CSS styling.
   It may break existing styling.
+* #9450: mathjax: Load MathJax via "defer" strategy
 
 
 Deprecated

sphinx/ext/mathjax.py

@@ -81,7 +81,7 @@ def install_mathjax(app: Sphinx, pagename: str, templatename: str, context: Dict
     domain = cast(MathDomain, app.env.get_domain('math'))
     if app.registry.html_assets_policy == 'always' or domain.has_equations(pagename):
         # Enable mathjax only if equations exists
-        options = {'async': 'async'}
+        options = {'defer': 'defer'}
         if app.config.mathjax_options:
             options.update(app.config.mathjax_options)
         app.add_js_file(app.config.mathjax_path, **options)  # type: ignore

tests/test_ext_math.py

@@ -71,7 +71,7 @@ def test_mathjax_options(app, status, warning):
     app.builder.build_all()
 
     content = (app.outdir / 'index.html').read_text()
-    assert ('<script async="async" integrity="sha384-0123456789" '
+    assert ('<script defer="defer" integrity="sha384-0123456789" '
             'src="https://cdn.jsdelivr.net/npm/mathjax@3/es5/tex-mml-chtml.js">'
             '</script>' in content)