_content/blog: fix video embed overflows on mobile

Fixes golang/go#42493

Change-Id: Ib2cf3802e3067578a9ece950033d6f547bd1828d
Reviewed-on: https://go-review.googlesource.com/c/website/+/677815
Reviewed-by: Dmitri Shuralyov <[email protected]>
LUCI-TryBot-Result: Go LUCI <[email protected]>
Reviewed-by: Dmitri Shuralyov <[email protected]>
Reviewed-by: Cherry Mui <[email protected]>

Author: seankhliao

_content/blog/default.tmpl

@@ -62,7 +62,7 @@
 {{end}}
 
 {{define "video url width? height?"}}
-<div class="iframe">
-  <iframe src="{{.url}}" width="{{or .width 560}}" height="{{or .height 315}}" frameborder="0" allowfullscreen mozallowfullscreen webkitallowfullscreen></iframe>
+<div class="iframe" style="aspect-ratio: {{or .width 560}} / {{or .height 315}}">
+  <iframe src="{{.url}}" width="100%" height="100%" frameborder="0" allowfullscreen mozallowfullscreen webkitallowfullscreen></iframe>
 </div>
 {{end}}