The amount of local storage needed for this guide is quite reasonable on modern devices, but it is still an intense experience for some browsers. Hopefully these notes will prevent frustration on your end.
These Safari notes were written based on my testing in 2020. Its support for local storage may have improved since then.
Downloading files for local offline use will continue in the background while you browse the guide (including this page). However, downloading is likely to pause in any of the following cases: Your device goes to sleep. (You might need to occasionally touch your mobile screen if it would otherwise time out due to inactivity.) You switch to your home screen or a different app. You browse to a different site.
If the downloading process is paused, just click the green button again to resume.
Warning: iOS 11 through iOS 13 have a known bug that crashes the downloading process in any of the above cases. If it does crash, try closing the tab, waiting 10 seconds, and then returning to the Guide. I don’t know if that will work, but it sounds plausible. This iOS bug is fixed in iOS 14. Web apps are not supported at all prior to iOS 11.1.
On MacOS, it should be sufficient to keep the Guide tab visible (not minimized), even if another window has focus.
Safari doesn’t allow web apps to monitor the space being used or the storage limit. Safari generally allows a web app to use 1 GB+, so the guide will fit as long as you have space available.
Safari places a time limit of ~7 days on local data. Re-visiting the Guide will refresh the time limit.
Saving the Guide to your home page might remove the time limit. To save to your home page, tap the browser icon with an arrow pointing out of a square (normally used for sharing a page). Then scroll down (not right) and select “Add to Home Screen”.
If you’ve read down this far, congratulations. You might be the kind of person who runs their browser in Private Browsing mode. As usual, all of your data from Private Browsing will be deleted when you leave Safari. So you’ll need to disable Private Browsing before downloading the Guide’s local files. And don’t forget to disable Private Browsing when downloading updates later.
Because of the strange way that browsers work, every page load will attempt to fetch ~25 KB from the internet even when the site is using its local offline copy. (E.g. viewing 40 pages will fetch ~1 MB.) If you have a large or unlimited data plan, this is nothing to worry about. If you have a very limited data plan, consider putting your phone in airplane mode while using the site in offline mode.
Of course, external links will still fetch from the internet as usual. Most external links are styled and colored in the usual way for your browser (e.g. blue and underlined). Links to Jepson are in orange, and also must fetch from the internet. Placeholder boxes that link to photos at CalPhotos or Calflora are also external links. By contrast, internal links to taxons are colored brown, green, or black and use the offline copy, as do all links from a thumbnail image to the full-sized image. A few miscellaneous internal links (not to a taxon) are also in the default browser style.
Return to the Guide home and give offline mode a try. If it doesn’t work, online browsing remains supported as always.