I know Readera as a e-reading android app, who doesn't respect ebooks internal css. I've tested a couple of my epub on the KOreader and Readest apps on android, just to be sure they worked ok, and they do, except for the title page getting a bit wonky because I use the flex css property, nothing worth troubling myself over.
If you use that Readera, I'd suggest deleting it and using a better app like one of the two I mentioned.
Readest is in the google play store, and is probably the one you'd prefer.
KOReader isn't, but if you go to this website with your phone, you can go to
their github and get their latest arm64.apk file.
No need to thank me for the upgrade in reading experience, love to help.
And I too, had the displeasure of useing Readera in the past; before I looked beyond the google play store most advertised apps, which are crap like Google

.
Edit: I'm not even sure ReadEra is the problem though.
I'm just sure the ebooks themselves are fine and 'broadly-enough' compatible. the images are contained cleanly inside, and are even generally jpg, rather than webp. So that's not the issue.