Kara new

gistfile1.kt

open class TagAttributes
    var TagAttributes.id = ""

class Tag<out T: TagAttributes>(val containingTag: Tag<TagAttributes>?, val t: T) {
    val attr: T = t
    fun attr(f: T.() -> Unit) {

    }
}

class A: TagAttributes()
    var A.href = ""

class INPUT: TagAttributes()
    var INPUT.value = ""

class TABLE: TagAttributes()
class TR: TagAttributes()
class TD: TagAttributes()

fun Tag<TagAttributes>.a(f: Tag<A>.() -> Unit) {}
fun Tag<TagAttributes>.input(f: Tag<INPUT>.() -> Unit) {}
fun Tag<TagAttributes>.table(f: Tag<TABLE>.() -> Unit) {}
fun Tag<TagAttributes>.tr(f: Tag<TR>.() -> Unit) {}
fun Tag<TagAttributes>.td(f: Tag<TD>.() -> Unit) {}


fun main(args: Array<String>) {
    Tag<A>(null, A()).table {
        tr {
            td {
                attr {
                    id = ""
                }
                input {
                    attr.value = ""
                    attr {
                        value = ""
                    }
                    a {
                        attr {
                            href = ""
                            id = ""
                        }
                    }
                }

            }
        }
    }
}

Did you mean
fun Tag<TR>.td(f: Tag<TD>.() -> Unit) {}
instead of
fun Tag<TagAttributes>.td(f: Tag<TD>.() -> Unit) {}