可选参数表示应变值，如果命名属性用于失踪，或它的价值不能被解析到特定类型或属性是无效或超出范围。

Note that the default value need not be of the type given, if the attr() expression is the entire property value.

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可选参数表示应变值，如果命名属性用于失踪，或它的价值不能被解析到特定类型或属性是无效或超出范围。

The optional argument represents a fallback value, which is used if the named attribute is missing, or its value cannot be parsed into the given type or is invalid/out-of-range for the property.

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