As well as can be measured 为评注性状语，是作者对主句内容 rural standards of living improved in the Fifth Reign 的解释、说明，即农村生活水准改善达到了可以测量出的程度。其中的well为程度副词。

至于as比较分句为什么没有主语，这是比较分句的结构特点决定的。比较分句省略主语而保留谓语动词通常见于以下几种情况：

1 比较对应项在比较分句中作主语。此时主语是必须省略的。例如：

More people came than (were/hand been) invited.

2 比较分句的主语为嵌入分句，而比较对应项在嵌入分句中做成分，此时作主语的嵌入分句可以省略。例如：

He came as early as (was/had been) expected.

= He came as early as (it) was/had been expected (that he would)

作主语的嵌入分句可以是限定分句，也可以是非限定分句（不定式分句或v-ing分句）

网友的这个句子中as后省略的是动名词分句（即动名词复合结构）standards of living improving。

其实，对于初学英语者而言，没有必要去追究语法解释，而只需知道as...as can be done是一种很常见的结构即可。例如以下COCA语料库例句：

You have taken vengeance about as far as can be justified.

As far as can be ascertained, Report X contained at most very little and possibly no specific evidence of chemical and biological agent production.

As far as can be judged from the net effect on happiness, the economic gains and psychological costs broadly offset each other, although the evidence on this is currently sketchy.

So what follows is the truth for now, as far as can be determined.

这些比较分句中省略的主语都是嵌入分句。