diff --git a/convert.rb b/convert.rb
index 1b6433c..bbcbd00 100644
--- a/convert.rb
+++ b/convert.rb
@@ -3,6 +3,7 @@ require "nokogiri"
 require "net/http"
 require "json"
 require "cgi"
+require "optparse"
 
 class KatexRenderer
   def initialize(source)
@@ -34,10 +35,26 @@ class KatexRenderer
   end
 end
 
+# Provided via this project's Gemfile
 ExecJS.runtime = ExecJS::Runtimes::Duktape
 
-renderer = KatexRenderer.new(Net::HTTP.get(URI("https://static.danilafe.com/katex/katex.min.js")))
-files = ARGV[0..-1]
+katex = nil
+OptionParser.new do |opts|
+  opts.banner = "Usage: convert.rb [options]"
+
+  opts.on("--katex-js-file=FILE", "Use the given KaTeX JS file to process LaTeX") do |f|
+    katex = f
+  end
+end.parse!
+files = ARGV
+
+if katex
+  katex = File.read(katex)
+else
+  katex = Net::HTTP.get(URI("https://static.danilafe.com/katex/katex.min.js"))
+end
+
+renderer = KatexRenderer.new(katex)
 files.each do |file|
   puts "Rendering file: #{file}"
   document = Nokogiri::HTML.parse(File.open(file))