I dislike the idea greatly.
It suggests a set dress code which guests must follow, as opposed to helpful considerations to assist the guests in choosing their own attire, as adults do.
So for outdoor weddings, having some reminders on the web site that guests will be walking on grass, or sitting in the sun and you mention this to ensure the guests dress for their comfort - that sounds like a great idea, and helpful.
Some folks like to be on the dressy end of the spectrum, no matter the spectrum, others on the comfort-over-fashion end. Some like bright colors, some neutral, etc. Some of the examples at the link left no room for individual preferences of this sort, in the photos/text, which struck me as dictatorial rather than considerate guidance.
If you wanted to convey something more as guidance than a phrasing of "afternoon casual" or any other descriptor, I still think it better to do so with some simple, helpful statements that won't read as mandates, and I would completely avoid any "don't wear this" guidance. Where someone is having a wedding in a venue/church that may *have* a dress code, such as covered shoulders, that information can be passed along and is not the same as anything suggesting that the happy couple require their guests comply with a particular aesthetic.
I don't mean to suggest you actually want to dictate your guest's attire, just that the "wear this, not that" photo idea for a wedding site seems to me to be condescending more than whimsical or uniformly (pun intended) useful.