If all someone knows is "how a web page functions (from a UX perspective)", then they don't actually know anything meaningful. If all they understand is that a link is clickable and that a web page is sent over the Internet, then they don't understand nearly enough to be telling someone else what to implement. HTML and CSS are relatively simple. A decent web designer should be able to build a mockup using them.
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If all someone knows is "how a web page functions (from a UX perspective)", then they don't actually know anything meaningful. If all they understand is that a link is clickable and that a web page is sent over the Internet, then they don't understand nearly enough to be telling someone else what to implement. HTML and CSS are relatively simple. A decent web designer should be able to build a mockup using them.