Can I add a custom attribute to an HTML tag?

Can I add a custom attribute to an HTML tag like the following?

<tag myAttri="myVal" />

You can add custom attributes to your elements at will. But that will make your document invalid.

In HTML 5 you will have the opportunity to use custom data attributes prefixed with data-.

You can amend your !DOCTYPE declaration (i.e. DTD) to allow it, so that the [XML] document will still be valid:

<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN"
"http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd"
[
  <!ATTLIST tag myAttri CDATA #IMPLIED>
]>

#IMPLIED means it is an optional attribute, or you could use #REQUIRED, etc.

More information is in DTD - Attributes.

No, this will break validation.

In HTML 5 you can/will be able to add custom attributes. Something like this:

<tag data-myAttri="myVal" />

Can I add a custom attribute to an HTML tag?

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