In some area it is almost impossible to write complete specs.
CSS is originally designed to be a end-user spec language to change the website as they like. But layout is so complicated that it take a lot of experience to define everything in position dynamically as requested.
And now people are using programs (js) to replace the job of CSS, to some extent giving up on writing specs for layout, even with years experience on it.
The hard thing is not the language itself, it is often the domain.
CSS is originally designed to be a end-user spec language to change the website as they like. But layout is so complicated that it take a lot of experience to define everything in position dynamically as requested.
And now people are using programs (js) to replace the job of CSS, to some extent giving up on writing specs for layout, even with years experience on it.
The hard thing is not the language itself, it is often the domain.