Skewer Gets HTML Interaction

A month ago Zane Ashby made a pull request that added another minor mode to Skewer: skewer-html-mode. It’s analogous to the skewer-css minor mode in that it evaluates HTML “expressions” in the context of the current page. The original pull request was mostly a proof of concept, with evaluated HTML snippets being appended to the end of the page (body) unless a target selector is manually specified.

This mode is still a bit rough around this edges, but since I think it’s useful enough for productive work I’ve merged it in.

Replacing HTML

Unsatisfied with just appending content, I ran with the idea and updated it to automatically replace structurally-matching content on the page when possible. Zane’s fundamental idea remained intact: a CSS selector is sent to the browser along with the HTML. Skewer running in the browser uses querySelector() to find the relevant part of the document and replaces it with the provided HTML. This is done with the command skewer-html-eval-tag (default: C-M-x), which selects the innermost tag enclosing the point.

To accomplish this, an important piece of skewer-html exists to compute this CSS selector. It’s a purely structural selector, ignoring classes, IDs, and so on, instead relying on the pseudo-selector :nth-of-type. For example, say this is the content of the buffer and the point is somewhere inside the second heading (Bar).

    <div id="main">
      <p>I am foo.</p>
      <p>I am bar.</p>

The function skewer-html-compute-selector will generate this selector. Note that :nth-of-type is 1-indexed.

body:nth-of-type(1) > div:nth-of-type(1) > h1:nth-of-type(2)

The > syntax requires that these all be direct descendants and :nth-of-type allows it to ignore all those paragraph elements. This means other types of elements can be added around these headers, like additional paragraphs, without changing the selector. The :nth-of-type on body is obviously unnecessary, but this is just to keep skewer-html dead simple. It doesn’t need to know the semantics of HTML, just the surface syntax. There will only ever be one body tag, but to skewer-html it’s just another HTML tag.

Side note: this is why I strongly prefer to use /> self-closing syntax in HTML5 even though it’s unnecessary. Unlike XML, that closing slash is treated as whitespace and it’s impossible to self-close tags. The schema specifies which tags are “void” (always self-closing: img, br) and which tags are “normal” (explicitly closed: script, canvas). This means if you don’t use /> syntax, your editor would need to know the HTML5 schema in order to properly understand the syntax. I prefer not to require this of a text editor — or anything else doing dumb manipulations of HTML text — especially with the HTML5 specification constantly changing.

When I was writing this I originally included html in the selector. Selector computation would just walk up to the root of the document regardless of what the tags were. Curiously, including this causes the selector to fail to match even though this is literally the page structure. So, out of necessity, skewer-html knows enough to leave it off.

For replacement, rather than a simple innerHTML assignment on the selected element, Skewer is parsing the HTML into an node object, removing the selected node object, and putting the new one in its place. The reason for this is that I want to include all of the replacement element’s attributes.

Another HTML oddity is that the body and head elements cannot be replaced. It’s a limitation of the DOM. This means these tags cannot be “evaluated” directly, only their descendants. Brian and I also ran into this issue in impatient-mode while trying to work around a strange HTML encoding corner case: scripts loaded with a script tag created by document.write() are parsed with a different encoding than when loaded directly by adding a script element to the page.

This last part is actually a small saving grace for skewer-css, which works by appending new stylesheets to the end of body. Why body and not head? Because some documents out there have stylesheets linked from body, and properly overriding these requires appending stylesheets after them. If body is replaced by skewer-html, all of the dynamic stylesheets appended by skewer-css would be lost, reverting the style of the page. Since we can’t do that, this isn’t an issue!

Appending HTML

So what happens when the selector doesn’t match anything in the current document? Skewer fills in the missing part of the structure and sticks the content in the right place. Next time the tag is evaluated, the structure exists and it becomes a replacement operation. This means the document in the browser can start completely empty (like the run-skewer page) and you can fill in content as you write it.

But what if the page already has content? There’s an interactive command skewer-html-fetch-selector-into-buffer. You select a part of the page and it gets inserted into the current buffer (probably a scratch buffer). The idea is that you can then modify and then evaluate it to update the page. This is the roughest part of skewer-html right now since I’m still figuring out a good workflow around it.

If you have Skewer installed and updated, you already have skewer-html. It was merged into master about a month ago. If you have any ideas or opinions for how you think this minor mode should work, please share it. The intended workflow is still not a fully-formed idea.

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Chris Wellons