It's not solid red. It has shading.
replyI don't think the blog post itself is using that emoji font. The screenshot on the Noto Emoji Github page[0] doesn't look like it's using any gradients for the heart emoji, just flat shading. But it is using gradients for some of the other emojis (e.g. the croissant), and obviously the SVG fallback is all or nothing, not per-glyph.
replyYou need to look closer; the heart emoji has a flat fill, but a gradient in its outline stroke, from lighter-than-red near the top, to darker-than-red on the bottom.
replyAs a bit more on this, until this piece I was oblivious to this COLRv1 thing, which is adding more of SVG-style functionality to already vector font standards.
replyhttps://developer.chrome.com/blog/colrv1-fonts
My natural cynicism is to ask "should a font really do this?" But I guess it's pretty neat.