Should similarity to HTML be the goal here? Markdown was designed to be easy to read, in direct contrast to HTML which isn’t particular easy for a non-developer to understand. The use of HTML seems like a fallback in the original spec. For other parts of the Markdown syntax, the syntax choice appears to be in opposition to HTML.
It’s worth asking why colons were chosen for some languages instead of equals signs. In particular YAML. As with Markdown, YAML was designed to be human readable first and foremost. Have a look at part of this example YAML document from Wikipedia:
receipt: Oz-Ware Purchase Invoice date: 2012-08-06 customer: given: Dorothy family: Gale
The syntax is quite readable to a human, spaced out with key/value pairs seperated as you might expect a list to be written down. Also, the colon is used in English to express “this is what I mean” whereas an equals sign is typically used to express something mathematical. E.g. John Gruber created a language: Markdown.