I recently ran into a situation where the easiest way to get some images to look the way they needed to was to use some PHP to add CSS classes based on the image characteristics.
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Every client theme I build uses a base theme, but it's not always the same base theme and for various reasons they don't always have the same breakpoints. I've always been a little bit bothered by having to duplicate breakpoints in JavaScript because it feels redundant and a potential source of future issues / inconsistency—on multiple occasions I've inherited sites where the breakpoints aren't consistent across CSS and JavaScript, leading to unexpected behaviour at certain browser widths. My solution to this has been to start including the breakpoints as JSON in the HTML source so that any JavaScript that needs them can find them there rather than having them hardcoded in the script itself.
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I've always been extremely pedantic about checking that things exist before trying to use them. Recently, thanks in part to some new features available in PHP 8+, I've learned that it's possible to write much more concise code that's equally robust.
Drupal's default RSS feed is bizarrely difficult to tame. There are a few issues with it, but the biggest one is the fact that each item's description has a link to the author's user profile appended to it in such a way that it can't be removed by any normal means—since user profile pages are not public for the vast majority of Drupal sites I build, this means there's effectively a link to a 403 error page in each RSS item.
Drupal's default oEmbed template is extremely sparse, and some accessibility checkers don't like that.
Tags
Determining which Drupal regions have content seems like a reasonably straightforward task, but, as evidenced by this 12+ year old, six-digit node ID issue, it is not. In the course of building a base theme for future Drupal projects I decided it was worth the time to see how well I could work around the problems referenced in that issue in order to get accurate region content flagging working.
Drupal's core “Manage display” functionality provides the option to link images to content, which is handy for, e.g., teasers. If the link contains nothing but an image, however, some accessibility checkers will (rightly) flag these links because there's no textual information indicating where they're going. One solution is to add a title attribute to the link.
Tags
I have a vocabulary for which the terms should always be sorted by their date field, so I wanted to disable manual sorting…