Don’t know about any complete guide. But tinkering with HTML and CSS can get you a long way. See e.g. https://www.zeitverschiebung.net/en/clock-widget https://

Don’t know about any complete guide. But tinkering with HTML and CSS can get you a long way. See e.g. https://www.zeitverschiebung.net/en/clock-widget https://www.zeitverschiebung.net/en/clock-widget for how to embed a clock widget on a web page.