Sometimes you want to paste a large region of code into a code block, and escaping the content is harder than fixing and start and end delimiters. This matters particularly in Markdown, where embedding large regions of code or text is common, whereas other languages you’d put it in its own file.

So I still suggest the ability to change the number of open and close brackets. Then you’ll also need an implicit newline or other way to distinguish content that starts with an open bracket.

The complication doesn't stem from lack of distinct start and end, what you are trying to solve for here, is when you have multiple languages in a single block, and want pretty colors on each. Seeing that HTML doesn't support imbrication of pre tags (or rather doesn't render one embedded in the next), that would probably not work without producing something that is not pure html.

> In a parser that would be much simpler to parse

Parsing a variable number of ` is not more complex than looking ahead for a closing boundary. In fact, once you introduce escaping characters, you need to handle escaping of the escaping character, which is slightly more complex.