Automatically convert HTML5 to HTML4

Published 27 Jan 2011

I love HTML5 and how easy things will become once it breaks through. However, it will take time for many browsers to support HTML5 and until they do, the code we write must be supported by older browsers as well.

Still, we want to write HTML5 code today, right? So, how do we solve this dilemma?

To make it possible to write HTML5 and still support older browsers, I created a class that automatically converts most new HTML5 elements to HTML4 elements when the response is sent to the client.

For instance, <nav>…</nav> is translated to:


for Internet Explorer 8 (which does not support HTML5), while Chrome and Firefox will receive the original HTML5 code (since they support it).

The basic implementation

First of all, HTML5 support is determined by one class, which implements a small interface. If the browser is considered to not support HTML5, the HTML5 code is converted to HTML4, using another small class.

The interface-based setup aims at making it easy to replace these quickly created implementations with better ones, if they prove insufficient. For instance, maybe you want to use a complete list of non-HTML5 browsers (here, we only consider IE8 and below to not have HTML5 support) and perhaps do not want the class name to be applied to the end tag (which the current implementation adds).

If you feel like improving these classes, feel free to send a pull request with a better implementation 🙂

The ugly test hack

When we finally did try the classes, we applied them in the Render event of a web form master page. It worked like a charm. When browsing the web site with Firefox, Chrome etc. we received the HTML5 code:

Firefox and Chrome gets HTML5

As you can see in the screenshot above, we get the original HTML5 code. However, in Internet Explorer 8, we get the converted HTML4 code:

Internet Explorer 8 gets HTML4

When viewing the site in IE8, we get converted HTML4 code. Isn’t this marvelous?

But wait a minute! Applying this in the Render event of the master page? That is, really bad, isn’t it? Yes, you are right. As I wrote, we were just trying it out. Relax…then read on 🙂

The pretty final solution

I guess that we can all agree that we want to make this conversion independent of our code. The solution above (applying it in the Render event) only works for Web Forms-based solutions. What if we want to automatically convert HTML5 to HTML4 in an ASP.NET MVC-based web site?

We need to find a better way of triggering the HTML5 conversion. So, how do we do it “correctly?

I decided to create an HttpModule that applies an HTML5 conversion filter to the outgoing response. This will automatically handle all outgoing responses, if the module is registered in web.config.

To enable the module, actions differ depending on which IIS setup you are using.

If you are using IIS 7.0 running in Integrated mode, you must add this following tag to the web.config system.webServer modules section:

<add name="NExtra.Web.HttpModules.Html5ElementConvertHttpModule" type="NExtra.Web.HttpModules.Html5ElementConvertHttpModule, NExtra, Version=, Culture=neutral" />

If you are using IIS 6.0 or IIS 7.0 running in Classic mode (this also applies to the dev server), add the following to the web.config system.web httpModules section:

<add name="NExtra.Web.HttpModules.Html5ElementConvertHttpModule" type="NExtra.Web.HttpModules.Html5ElementConvertHttpModule, NExtra, Version=, Culture=neutral" />

That should be all you have to do to enable automatic HTML5 to HTML4 conversion.

All the best!