This is kind of a dilemma.

The top priority concern should be that the output is flexible for different kind of customization. Usually people using `--split-page 1` do not use the HTML file generated by pdf2htmlEX, what they need are the css file and the `.page` files. So I've been hoping that each single `.page` file is holding all information necessary for the corresponding page. Therefore I don't want to remove the current  content from `.page` files.

So now I'm heading towards the JS solution. Ideally we have all the metrics packed into a JSON, in some way like:

```javascript
[{
   urls : ['1.page', '2.page', ... , '10.page'],
   width: 100,
   height: 200,
   ctm: [...]
},{
   urls: ['11.page'],
   width: 200,
   height:100,
   ctm:[...]
}]
```

I think it's reasonable since very few PDF have different sizes or ctms in the pages.

In this way, JS will draw some empty frames while loading the pages. But what concerns me is that the old code seems to be not flexible enough for this situation. And using `.pd`, `.pf` etc. look more like a hacking to me, which would become difficult to maintain once the page layout is changed. I see you have added `loaded` for the `Page` class, and I think we should introduce different behavior based on `loade`, or even create a new class, e.g. `PageLoader`, instead of creating a fake `.pf` etc.

What do you think?