In the last year or so, I took on a project to learn the intricacies of SVG, more specifically SVG paths and hand-coding them. I was inspired by the idea when I saw Brent Jackson's Geomicons, a minimal iconset built from the ground up for performance.
It's great, I can drop these few readable lines of code in my document, shave off a request, easily adjust the color and size with a single attribute or class, and modify any of these properties since they are accessible through the DOM. The flexibility is fantastic!
After experimenting more, I learned SVG stroke is another property definitely worth utilizing... and perfect for line icons. Not only can you limit the amount of points on a path without having to outline a shape, you can simply change the weight with stroke-width as well as the shape of the line caps and the line joins with stroke-linecap and stroke-linejoin.
I ended up creating 84 line icons using strictly stroke to define the shape. Each icon is hand-coded along a 32x32 grid with a 2 point margin allowing for a maximum of 4px (or 12.5%) stroke-width. 4px seems small but it's relative to the 32x32 grid. I tried to place as few points as possible to keep the shapes simple but also to reduce on file size. All icons measure in at a total of 9KB minified (2KB in SVGZ).
<svg viewBox="0 0 32 32" width="128" height="128" fill="none">
<path stroke="SlateGray" stroke-width="1px" stroke-linejoin="round"
d="M2 4 L30 4 30 22 16 22 8 29 8 22 2 22 Z" />
</svg>There are a few caveats
One of the bigger caveats is the antialiasing, especially for screens without subpixels. If you want to pixel-hint as much as possible, I recommend using stroke-width increments of 0.5px or 1.5625%. Regardless of what size you set the stroke-width, I recommend using overflow: visible incase the linecap edge falls just outside the viewBox.
By nature line icons are not meant to be tiny. I recommend not going any smaller than 24px in width/height.
You do miss out on caching when using these icons inline, but since they have such a small footprint I believe it's ok for them to be loaded with the HTML each time. Some people may disagree, but it really comes down to what kind of site you have and how many icons you plan to use. If you do want to utilize the cache, you are able to link the SVG externally and then render the icon with SVG use. You can learn more about the use tag on CSS-Tricks.