Execute JS Function After Page Loads ?

I have long html/CSS (no CMS or php, etc.) web pages. Some people run away when they see too much content on a page. So I hide the large lower section of the pages and have the user click anchor text to execute simple JS code to change the display from 'none' to 'block' or back to re-hide.

Code:

(BTW my research indicates that search engines do not know the difference between display='none' and normal content, or I would not be hiding content in the first place.)

Within this hidden content, I have anchor text to provide name handles

Code:

So users can click a link and go directly to that section. Within that link I include

Code:

So that works fine within a particular page. But when I want to link to an anchor on another page which is within this hidden content, the browser cannot display the anchor text because it is still hidden.

Is there a way to execute the JS code to change the DOM, changing the display to 'block' on the content of the new page?

I do not see how, because the new page isn't loaded yet, there is no DOM within the browser yet, so how can it be manipulated?

Of course if I was using php I could generate the code with the property set as desired. But for straight up html/CSS/JS pages is this possible?

I get the feeling I am missing something simple here, either a simple solution or the simple fact that this is just an inherent limitation of html.