These instructions are slightly different for each version of macOS. Please see the User Tip appropriate for your macOS version.
Setting up a local web server on MacOS X 10.6 "Snow Leopard"
Setting up a local web server on MacOS X 10.7 "Lion"
Setting up a local web server on OS X 10.8 "Mountain Lion"
Setting up a local web server on OS X 10.9 "Mavericks"
Setting up a local web server on OS X 10.10 "Yosemite"
Setting up a local web server on OS X 10.11 "El Capitan"
Setting up a local web server on macOS 10.12 "Sierra"
Setting up a local web server on macOS 10.13 "High Sierra"
Setting up a local web server on macOS 10.14 “Mojave”
Setting up a local web server on macOS 10.15 “Catalina”
Setting up a local web server on macOS 11 “Big Sur”
Setting up a local web server on macOS 12 “Monterey”
Setting up a local web server on macOS 13 “Ventura”
Setting up a local web server on macOS 14 “Sonoma”
Setting up a local web server on macOS 15 “Sequoia”