What is ethics?
Ethics is a field of discourse dealing with what is good practice, what is bad practice and so forth. It's informed by people's morals, which are individually held beliefs about good and evil.
Thus it is pretty much impossible to give any definitive or authoritative answer on any ethical question, due to the nature of ethics. The best you can get is "A lot of people think that ...".
That is, if you really are asking about ethics, you are asking for primarily opinion-based answers. However, I suspect this isn't entirely the case.
Your question doesn't appear to be about ethics
Whether you may copy someone else's work is covered by Copyright law. That is, it's not a field where ethics is really much of a consideration unless you have specific reason for ignoring the laws (eg whether it's okay to ignore a certain law may itself be an ethical question).
However in this case it clearly looks like what you're asking about is covered by Copyright law.
Under Copyright law, any copying of someone else's work is restricted. By copying, it means any methodical effort to duplicate someone's work, in whole or in part, for any work that can be protected by copyright. A design can be protected by copyright, therefore any methodical effort to duplicate someone's design is restricted under copyright law.
What does restricted mean?
Simply put, restricted means you don't have a right to copy the work unless you have the owner's permission, or your use of the work falls under "fair use" or "fair dealing", a set of exemptions to copyright restrictions which varies by country.
What does copying mean?
Under copyright law, simply coming up with a similar design by accident or coincidence is not copying, and nor is borrowing an idea but not methodically reproducing it from the author's work.
Examples of things that are copying:
Taking a screenshot of the design, putting that in a Photoshop layer and creating your own design in a layer above it, using the layer below as a template for all or part of your design.
Looking at the HTML or CSS source code of the design, copying and pasting parts of it into your own source code, possibly modifying parts of it.
Examples of things that aren't copying:
Looking at their design to get a sense of what design ideas or techniques look good, and creating your own design which may make use of similar ideas without any effort to methodically trace, copy pixel-by-pixel or dimension-by-dimension, etc.
Looking at the HTML or CSS source code of the design in order to figure out how they have achieved a certain effect. Use those same concepts in your own source code to achieve similar effects, writing the code yourself without copying and pasting or any word-by-word/letter-by-letter duplication.
Being a civil law, there is no public prosecution of copyright infringement - it is up to the copyright owner to prove that you infringed and it is up to you to prove that either you didn't, or that your copying falls under the "fair use"/"fair dealing" exemptions in your area.
One common fair use exemption in many places is copying for the purpose of reviewing someone's work, eg reviewing someone's design and showing a small screenshot of the design to illustrate your review. Another is copying for the purpose of parody, and so on.
Don't take any of this to mean that designers don't infringe copyright, or that copyright infringement is inherently evil (that second point would take us full circle back to a question of ethics). I would go so far as to say that on the web, copyright infringement is commonplace, even. It comes down to whether your personal morals align strictly with copyright law or whether you are willing to take a more risky approach.