how split a mkv with subtitles

You can use mkvtoolnix, then go to -> output, and -> splitting -> by parts based on timestamps -> and there you can write the beginning and the end of the poart you want extract (without spaces: like 1:02:07-1:04:40, for example).

show “alt” attribute after an image

You can use a javascript, like this

<script type="text/javascript">
// img[style]').each(function(){
       $el = $(this);
       var style = $el.attr('style');
       $el.attr('style','');
       $el.parent().attr('style',style);
    }); //Moves the inline styles

      $("img").each(function(){
          var title = this.alt;
          $(this).after(''+ title +'');
      }); //Adds the dynamic captions.
 });
 //]]>
</script>