Yes, it would be OK to do that. One of the worst things abstractions can do is seal you away from the lower level even though you still need to get there. Another example I recently used was a CRUD framework that 100% took over the routing of URLs and had no callouts whatsoever for a non-CRUD-framework page. A CRUD framework may do many wonderful things for my CRUD pages but I may still need a URL to go somewhere else.

"Why? Not being snarky, I'm genuinely trying to understand this better."

It's almost a definitional issue, honestly. If you aren't doing some sort of limiting and building on limitations, you don't have an abstraction, you just have a library. A library is just a whole bunch of programming you could have done yourself, but doesn't impose any further restrictions on you. A classic example would be an image decoding library. They generally do not impose anything on you. They're just huge bits of code that turns images into pixels. You justifiably pull one in because why would you want to write that code again when battle-tested code already exists, but it doesn't impose anything on your code.

Now, when you start getting an "abstraction" that tries to make it so you can operate on multiple types of images at once, you're going to have some restrictions. Your abstraction is going to make some choices about the color spaces you can operate on. Your abstraction is going to make choices about what features they expose from the underlying graphics formats. For example, consider how a generalized "operate on all images" library will handle animated formats. It's a very sensible answer to say it won't operate on them at all. Or perhaps it offers animation support and errors out if you choose a format that doesn't support them. Can it export SVGs? Can it export SVGs with any sort of embedded Javascript? Does it do anything sensible with imported SVGs with embedded Javascript? What does it do with vector versus pixel based images?

You may say "ah, but jerf, I can easily provide a library that simply allows all those things no matter what... or, well, at least I can hypothesize such a library, providing it would actually be a pain... but there's no reason it couldn't exist". Exist, yes, but what you'd find is that if it let you do everything to every image format that it actually wouldn't do much! You can provide a library that takes a file, figures out what kind of image it is, and hands back a specific instance of a *JPEG or *GIF or *TIFF or *SVG, but if your supposed abstraction lets you do anything with anything, if you sit down and work out what it actually means for my JPEG support to give you access to the DCT tables directly and the GIF library to support the exact GIF animations and SVGs to offer direct manipulation of the SVG DOM and PNGs to offer direct access to all the frame elements in a PNG is that you don't have an "abstraction" anymore! Your "abstraction" is just a whole bunch of libraries bundled together with a thin abstraction around loading a file, and programmers using it this way get no ability to work across file types because they all have a completely different API.

And then you say "But I could offer a subset", which is 100% true, and that would be an abstraction. But then that abstraction would offer no access to PNG frames or JPG DCT tables.

And that's fine. There's no law whatsoever that a single "thing" must either be an abstraction or a library. It would be perfectly sensible to offer a total image solution that had a series of abstractions of varying level of restrictions and varying corresponding levels of power, and also include a full library for deep manipulation of every individual type. It's fine to offer a generalized "Image" type that has a ".ConcreteImage()" method that returns a concrete image that can be used with the libraries and "penetrate the abstraction". My point here is about the fact that if you want more "power", to operate on multiple image types with the same code, to take advantage of simply committing to a page-based interface to the Web and thereby being able to take advantage of the resulting simplifications it offers, to be able to plug arbitrary compression algorithms on to an IO stream, you always have to build these things on a reduction of power of some sort because you can't get any "abstractive" power if you're trying to offer everything to everybody. It's a fundamental fact of abstractions, it's precisely where they get their power from in the first place, and there is no option where you get those benefits without paying the price.