I want to download a HTML code from this website: "http://www.cssscript.com/pure-css-floating-labels-for-text-fields/", the license is "Unknown", this mean that I can edit and add the code the my website without write any License text in the edited code? In the website which I want to download the code I found this sentance:" Inspired by Matt D. Smith’s design and created by skielbasa."
On the CodePen blog you will find that any Pen that is made public is automatically MIT licensed:
By default, Pens you save on CodePen are public. PRO accounts have the ability to save as private, so for information on that jump down to the next section. Public pens are automatically MIT licensed. We include it on the Details View of your Pen and in the HTML source when the Pen is viewed in its "Full Page" view. But the license applies to the Pen itself, wherever it is viewed.
If you go to the details view for that particular pen you can see the copyright statement for that code as well as it's license.
In answer to your general question, you can't use someone else's code without the copyright holder's permission. The answer to your specific example is that it is not an unknown license. It was released under a known license and you may use it based on the license under which the author released it.
No, it means you can't copy it.
By default, the copyright to a work is owned by its creator, and nobody else is allowed to copy it, or create derived works, without their permission. That permission can be granted by a license. "License unknown" doesn't really tell us anything, but it certainly isn't clearly granting you permission. So you don't have permission to copy, and thus you cannot. You would have to seek permission from the copyright holder.
(Some jurisdictions do allow for "fair use" exceptions, which allow you to copy a work without permission. You haven't said what jurisdiction you are in.)
You can use the code - if you are willing to risk financial suicide. You don't know how the code is licensed. That means when the copyright owner finds out that you are using their code, you either accept that you committed copyright infringement, which could be very expensive, or you accept whatever the license was.
Now what you can do: You can read the code, you can understand it, you can learn from it, and when you do all that you will be a better software developer and you can write code (on your own, without copying the code that you found) that does exactly the same thing.