Karma/Anti-conventions

Not closing <img /> or <meta /> tags

You can usually get away with not closing these tags but they could cause weird rendering of elements that follow them. If you don't close <img>, the browser might think that any following elements are children rather than siblings and display them accordingly.

Using document.write()

Using document.write('<div>some html</div>'); instead of $('#parentDiv').html('<div> some html </div>');

document.write is just bad in general. Some browsers don't like it (chromium) and in all browsers it freezes all other execution while it is writing to the page.

Manually constructing an html sequence rather than passing a string

The former is not incorrect per se, but it is much, much slower than passing a string to .html(), at least according to the jQuery Cookbook.

  $parent = $(document.createElement('div'))
                 .appendTo('#someId')
                 .attr({id: 'foo', class: 'foobar baz'})
                 .text('hello world');

  According to jQuery Cookbook this is much, much faster
  	  
   $parent = $('#someId').append($('<div id="foo" class="foobar baz">Hello World</div>'));
   
FYI, prefixing cached jQuery reference w/ a "$" is a good practice.

Putting style information inline

  
   <div style="display:none;"> </div>
   

  This makes the lesson much harder to debug as you have to look both in the .css file
  and at the html. As the lesson grows, which it will, it gets harder and harder to 
  pin down small bugs

Mixing naming conventions, like camelCase and not_camel_case

Neither of these naming conventions is the "correct" convention but mixing them makes it hard for others to work w/ your code

  
   <div id="topbtn_right"> would be better written as <div id="topBtnRight">