The sad thing is, XML, and thus XHTML, has this. And it's nowhere near as ugly as this, which is saying something, because "lol, XML".
If what a person wants is cross-referenced elements in an XML document, then that's what they should use: https://msdn.microsoft.com/en-us/library/ms950811.aspx
If what a person wants is logic in their HTML, then they're standing on a slippery slope that ends in Classic ASP, PHP, and other wailings and gnashing of teeths.
The sad thing is, XML, and thus XHTML, has this. And it's nowhere near as ugly as this, which is saying something, because "lol, XML".
If what a person wants is cross-referenced elements in an XML document, then that's what they should use: https://msdn.microsoft.com/en-us/library/ms950811.aspx
If what a person wants is logic in their HTML, then they're standing on a slippery slope that ends in Classic ASP, PHP, and other wailings and gnashing of teeths.