Q:

Why does XhtmlTextWriter ignore custom attributes?

I've found some weird behaviour and I was wondering if anyone can help out here.
I'm creating a form using the XhtmlTextWriter class that inherits the addAttribute methods. I'm creating an input tag that needs a nice (HTML5) placeholder attribute. The addAttribute method has two parameters: the attribute name and the value. The attribute name can either be picked from the HtmlTextWriteAttribute enum or inputted manually as a string. Since 'placeholder' is not available in the enum, I used the following code:
StringWriter sw = new StringWriter();
XhtmlTextWriter html = new XhtmlTextWriter(sw);
html.AddAttribute(HtmlTextWriterAttribute.Type, "text");
html.AddAttribute(HtmlTextWriterAttribute.Name, "firstname");
html.AddAttribute("placeholder", "First Name");
html.AddAttribute("maxlength", "25");
html.RenderBeginTag(HtmlTextWriterTag.Input);
html.RenderEndTag();//input
return sw.ToString();

This nicely creates the element & attributes specified... EXCEPT for placeholder:
<input type="text" name="firstname" maxlength="25"></input>

Does anyone know where my placeholder is? (As you can see with maxlength, using a string for attribute name works...)
Note: This does work, but it's not so pretty:
html.WriteBeginTag("input"); 
html.WriteAttribute("type", "text");
html.WriteAttribute("placeholder", "First Name");
html.Write(HtmlTextWriter.SelfClosingTagEnd);

// Update: Same problem with the required attribute... Could it be something HTML5 specific?

A:

This is because you're using XhtmlTextWriter, which is strict with its attributes and won't write out unrecognised ones (due to the need to produce valid XHTML). You have two options.
One: Use HtmlTextWriter instead:
HtmlTextWriter html = new HtmlTextWriter(sw);

Two: If you need to use XhtmlTextWriter for some reason, you can add placeholder as a recognised attribute for input elements before you add the attribute to the element:
html.AddRecognizedAttribute("input", "placeholder");

