So there's 3 options you have for rendering the colours outside the sRGB space in this kind of image.

1. Don't. This is usually the most honest, and what all but the first diagram in this article opts for.

2. Clamping. You just set the green component to 255 for every colour beyond green=255, which effectively looks like you extend the edges of the triangle to the edge of the visual range. This is the most common, and the approach used in the article's first image, but it's basically a lie. Some articles will dumb the out of range colors to make it clear they're not the real colour, but this article's first image doesn't.

3. HDR: If the author uses an image format capable of decoding HDR data, and your browser, OS and monitor, and the author's authoring pipeline are all correctly configured to pass through that HDR data, you can get a bit more colour, depending on your monitor. Not the full visible gamut, but up to whatever colorspace your monitor is using.