Insert video HTML tag for images ending with ".webm"

Markdown elements like `![Alt video text](dir/video.webm)` will be inserted
with a video HTML element in a similar way to Gitlab markdown.

src/Writers/HTMLWriter.jl

@@ -1030,7 +1030,11 @@ mdconvert(h::Markdown.Header{N}, parent; kwargs...) where {N} = DOM.Tag(Symbol("
 
 mdconvert(::Markdown.HorizontalRule, parent; kwargs...) = Tag(:hr)()
 
-mdconvert(i::Markdown.Image, parent; kwargs...) = Tag(:img)[:src => i.url, :alt => i.alt]
+mdconvert(i::Markdown.Image, parent; kwargs...) = if endswith(i.url, ".webm")
+    Tag(:video)[:src => i.url, :width => "800", :controls => "true", Symbol("data-title") => i.alt]
+else
+    Tag(:img)[:src => i.url, :alt => i.alt]
+end
 
 mdconvert(i::Markdown.Italic, parent; kwargs...) = Tag(:em)(mdconvert(i.text, i; kwargs...))