You made this element optional for backward compatibility, but you'll notice that the optionality of this field is a good idea anyway, because not all addresses have apartment Numbers.
您可以使这个元素为向后兼容的可选项，您会发现这个字段的这个可选性是一个很不错的主意，因为并不是所有的地址都有房间号码。
XHTML V1.0 Strict, and Apache V2.2.12 on Ubuntu kernel 9.10. The examples provided should work with new versions of each element and in some cases are backward-compatible.
本文中提供的例子应该同样适用于它们的新版本，并且在某些情况下，还是后向兼容的。
By working backward through a decay sequence, the scientists are reasonably certain they created element 118, even though the highly unstable new atoms survived only a millisecond apiece.
通过回溯一个衰变序列，科学家们有理由确认他们创造了118号元素，即便这种高度不稳定的新元素仅仅存在了一毫秒。
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