ソースコードの右上に言語名を表示するやつ

こういう感じのスタイルシートを当てておいて,

pre.code {
  position: relative;
}

pre.code[data-lang-label]::before {
  content: attr(data-lang-label);
  font-family: 'Helvetica Neue', 'Helvetica', 'Arial', 'Hiragino Kaku Gothic Pro', 'Meiryo', 'MS PGothic', sans-serif;
  font-style: italic;
  position: absolute;
  top: 0;
  right: 0;
  background-color: #FFFFFF;
  border-radius: 0 0 0 4px;
  padding: 0 7px;
}

pre.code[data-lang-label]:hover::before {
  display: none;
}

雑なスクリプトをフッターとかに置いて pre.code に data-lang-label という属性をつけてやる.

<script type="text/javascript">
window.addEventListener("load", function onLoad() {
  window.removeEventListener("load", onLoad);
  var langLabels = {
    "actionscript": "ActionScript",
    "awk"         : "AWK",
    "bc"          : "bc",
    "brainfuck"   : "Brainfuck",
    "c"           : "C",
    "cabal"       : "Cabal",
    "cl"          : "Common Lisp",
    "clojure"     : "Clojure",
    "cmake"       : "CMake",
    "cpp"         : "C++",
    "cs"          : "C#",
    "css"         : "CSS",
    "coffee"      : "CoffeeScript",
    "crystal"     : "Crystal",
    "d"           : "D",
    "dart"        : "Dart",
    "elixir"      : "Elixir",
    "erlang"      : "Erlang",
    "fortran"     : "Fortran",
    "fsharp"      : "F#",
    "go"          : "Go",
    "haskell"     : "Haskell",
    "hatena"      : "Hatena",
    "haxe"        : "Haxe",
    "html"        : "HTML",
    "io"          : "Io",
    "lex"         : "Lex",
    "lhaskell"    : "Literate Haskell",
    "lisp"        : "Lisp",
    "lua"         : "Lua",
    "java"        : "Java",
    "javascript"  : "JavaScript",
    "json"        : "JSON",
    "julia"       : "Julia",
    "kotlin"      : "Kotlin",
    "make"        : "Makefile",
    "markdown"    : "Markdown",
    "objc"        : "Objective-C",
    "ocaml"       : "OCaml",
    "perl"        : "Perl",
    "perl6"       : "Perl6",
    "php"         : "PHP",
    "prolog"      : "Prolog",
    "python"      : "Python",
    "r"           : "R",
    "ruby"        : "Ruby",
    "rust"        : "Rust",
    "sass"        : "Sass",
    "sed"         : "sed",
    "scala"       : "Scala",
    "scheme"      : "Scheme",
    "sh"          : "Shell script",
    "sml"         : "Standard ML",
    "sql"         : "SQL",
    "svg"         : "SVG",
    "swift"       : "Swift",
    "tex"         : "TeX",
    "typescript"  : "TypeScript",
    "vim"         : "Vim script",
    "xml"         : "XML",
    "yacc"        : "Yacc",
    "yaml"        : "YAML",
    "zsh"         : "Zsh",
  };
  var codes = document.querySelectorAll("pre.code");
  var len = codes.length;
  for (var i = 0; i < len; i++) {
    var code = codes[i];
    var lang = code.dataset.lang;
    if (lang) {
      code.dataset.langLabel = langLabels[lang.toLowerCase()] || lang;
    }
  }
});
</script>