Improve HSX parser

[jeyj0]

• [ ] disallow elements as children of other elements that don't allow them per spec
  • html example: p tags only allow phrasing content as children
  • [ ] only allow svg elements and attributes inside <svg></svg>
• [ ] only allow attributes for elements where those attributes are valid
  • for implementing this, explicit lists of attributes could be opt-in until all elements are done, which would allow a gradual implementation

[CSchank]

I support this. I already love how HSX is much more type-safe than HTML itself 😊