I would have expected SVGs to be like PDFs and render the same across devices. Is the issue that some renderers don’t implement the full spec, or that some implement parts incorrectly?

They are like PDFs in that they do not render the same with different software or on different devices.

SVG can for example contain text elements rendered with a font. If the font is not available it will render in a different one. The issue can be avoided by turning text elements into paths, but not all SVGs do that.

More like HTML and getting different browsers to render pixel perfectly identical result (which they don't) including text layout and shaping. Where different browser don't mean just Chrome, Firefox, Safari but also also IE6 and CLI based browsers like Lynx.

PDFs at least usually embed the used subset of fonts and contain explicit placement of each glyph. Which is also why editing or parsing text in PDFs is problematic. Although it also has many variations of Standard and countless Adobe exclusive extensions.

Even when you have exactly the same font text shaping is tricky. And with SVGs lack of ability to embed fonts, files which unintentionally reference system font or a generic font aren't uncommon. And when you don't have the same font, it's very likely that any carefully placed text on top of diagram will be more or less misplaced, badly wrap or even copletely disappear due to lack of space. Because there is 0 consistency between the metrics across different fonts.

The situation with specification is also not great. Just SVG 1.1 defines certain official subsets, but in practice many software pick whatever is more convenient for them.

SVG 2.0 specification has been in limbo for years although seems like recently the relevant working group has resumed discussions. Browser vendors are pushing towards synchronizing certain aspects of it with HTML adjacent standards which would make fully supporting it outside browsers even more problematic. It's not just polishing little details many major parts that were in earlier drafts are getting removed, reworked or put on backlog.

There are features which are impractical to implement or you don't want to implement outside major web browsers that have proper sandboxing system (and even that's not enough once uploads get involved) like CSS, Javascript, external resource access across different security contexts.

There are multiple different parties involved with different priorities and different threshold for what features are sane to include:

- SVG as scalable image format for icons and other UI elements in (non browser based) GUI frameworks -> anything more complicated than colored shapes/strokes can problematic

- SVG as document format for Desktop vector graphic editors (mostly Inkscape) -> the users expect feature parity with other software like Adobe Illustrator or Affinity designer

- SVG in Browsers -> get certain parts of SVG features for free by treating it like weird variation of HTML because they already have CSS and Javascript functionality

- SVG as 2d vector format for CAD and CNC use cases (including vinyl cutters, laser cutters, engravers ...) -> rarely support anything beyond shapes of basic paths

Beside the obviously problematic features like CSS, Javascript and animations, stuff like raster filter effects, clipping, text rendering, and certain resource references are also inconsistently supported.

From Inkscape unless you explicitly export as plain 1.1 compatible SVG you will likely get an SVG with some cherry picked SVG2 features and a bunch of Inkscape specific annotations. It tries to implement any extra features in standard compatible way so that in theory if you ignore all the inkscape namespaced properties you would loose some of editing functionality but you would still get the same result. In practice same of SVG renderers can't even do that and the specification for SVG2 not being finalized doesn't help. And if you export as 1.1 plain SVG some features either lack good backwards compatibility converters or they are implemented as JavaScript making files incompatible with anything except browsers including Inkscape itself.

Just recently Gnome announced working on new SVG render. But everything points that they are planning to implement only the things they need for the icons they draw themselves and official Adwaita theme and nothing more.

And that's not even considering the madness of full XML specification/feature set itself. Certain parts of it just asking for security problems. At least in recent years some XML parsers have started to have safer defaults disabling or not supporting that nonsense. But when you encounter an SVG with such XML whose fault is it? SVG renderer for intentionally not enabling insane XML features or the person who hand crafted the SVG using them.

Even PDFs don't always render the same from one platform to another. I've mostly seen it due to missing fonts.

Most renderers don't implement the full spec.