@idered Thanks for sharing the tip. As a side comment, I thought it would be worthwhile to note the example of image tags that display non-content visual aids (loaders and such) are counteractive to the distinction drawn between CSS (the visual style of the document) and HTML (the data structure of the document).
<img> tag has the meaning, "This picture is consumable content that is relevant to the page." e.g. a photograph in a news article is a relevant and impactful piece of data that is related to the rest of the article. In this case the loader is not a relevant or impactful part of the actual content on the page, it is a user interface helper for those who aren't visually impaired.
That said, if semantics are a priority for any future reader, It might be slightly more semantic to have loaders be faceless
<div class='loader'> tags with background images.
Outside of that, thanks again for sharing the tip -- I believe you are using the hidden attribute correctly in your use case.