What are the actual problems of not closing tags and attributes in HTML

Recently a friend decided not to close his tags or attributes in HTML because it's not required and he'll save some bandwidth and download time. I told him it's a bad idea and to be "better safe than sorry", however, I could only really find 2 resources on the issue:

1. http://css-tricks.com/13286-problems-with-unquoted-attributes/
2. http://www.cs.tut.fi/~jkorpela/qattr.html

#1 is good, but even as he said, they aren't really real world examples, which is why I went to 开发者_运维技巧#2, but they only really show an <a> which is much more different than most other tags/nodes.

Is there another resource or test cases as to a better reasons to quote your attributes and close your tags?

You can often leave the closing tags off many elements without changing 'the way it looks'. However, even though one of the main goals of HTML5 is to standardize how browsers deal with bad markup, not closing tags can impact your content in unexpected ways. Here's a simple example, a list of items where some of the items are blank, both without explicitly closed tags and with:

<ul>
    <li>Item
    <li>
    <li>Item
    <li>
    <li>Item
</ul>

<ul>
    <li>Item</li>
    <li></li>
    <li>Item</li>
    <li></li>
    <li>Item</li>
</ul>

Looking at the two in a browser they look identical. However, if you add a bit of CSS to hide the empty ones:

li:empty { display: none; }

Now they don't look the same, even though the markup hasn't changed from the previous example. The underlying reason for this is that the two versions produce different DOM trees, this version iterates through all the nodes in both lists and counts them, then shows the results and the list of nodes found in alerts. You can see the top list has 12 DOM nodes, the lower list has 15. The results are at least consistent cross browser, and the difference is in text nodes which you'll frequently skip over when scripting anyway, but this shows that even if the visual output looks the same when tags are closed or not, there are underlying differences even in an example as simple as this.

Not closing tags can lead to browser incompatibilities and improperly rendered pages. That alone should be enough reason to properly close your tags.

Saving bandwidth and download time is a horrible excuse, if you ask me. It's 2011, and even on dialup the few bytes you save on not closing a few tags will not be even close to noticeable. A mangled page due to improper rendering, however, will be.

Not closing tags can create unexpected blank spaces between elements in the markup.

Consider the following example.

<!-- English quoting rules. -->
<style>
    blockquote > p::before { content: open-quote; }
    blockquote > p::after { content: no-close-quote; }
    blockquote > p:last-of-type::after { content: close-quote; }
</style>

<!-- This is ok. -->
<blockquote>
    <p>Lorem ipsum dolor sit amet.</p>
    <p>Lorem ipsum dolor sit amet.</p>
</blockquote>

<!-- This isn't. There are blank spaces after the last (unclosed) <p> element.
     Thus, the closing quote appears separated with a space from the text. -->
<blockquote>
    <p>Lorem ipsum dolor sit amet.
    <p>Lorem ipsum dolor sit amet.
</blockquote>

That's just bad coding practice in my opinion.

There are two types of programmers; those that care, and those that don't.

It's lazy programming, the same as not having coding standards, or not formatting your code... it's like being a carpenter and not sanding the edges of the table you just built.

Most browsers support it, but some might complain.

Most HTML tags are containers. Consider:

<style>
 section {
   color: red;
 }
</style>
<section>
  Stuff inside a section
</section>
Stuff outside a section
<p>
  Other text
</p>

In this example, "Stuff inside a section" would be red text, "Stuff outside a section" is not red. In this example:

<style>
 section {
   color: red;
 }
</style>
<section>
  Stuff inside a section

Stuff outside a section
<p>
  Other text
</p>

... in this example, "Stuff inside a section", " Stuff outside a section", and "Other text" would ALL be red - that is, the section never ended. The browser may try to assume where the section could have ended, but in my above example the only assumption possible is that the section continues to the end of the document, which is not what was intended.

In short, not closing HTML tags just makes things more confusing for you, will cause pages to render inconsistently from expectations and between browsers, and is just generally a bad idea. So bad, in fact, that it shouldn't even be taken as a serious suggestion at all. Your friend has clearly never developed an actual web site.

You can't do much in terms of site design/layout if you don't close tags.