Fabric

Fabric is a lightweight modloader that also has a separate API required in the mods folder. It has been worked on alongside the 1.14 snapshots of Minecraft Java Edition since before snapshot 18w49a (before December 5th 2018). Not much is known of its creation prior to this.

It is currently being supporting for Release 1.14.4 but some snapshot builds exist for 1.15 (as of writing this 19w34a and 19w35a).

Known installation methods of Fabric
MultiMC was the first launcher (albeit third-party) to support the Fabric Modloader.

As of August 11 2019/0.6.7 update (around this time) MultiMC announced and added in an update to the launcher with an 'Install Fabric' button to allow for easier installation of the modloader compared to its prior zip importing method required before. Forge 1.14 support is not available for the launcher currently due to the current state of the modloader and MultiMC being unable to handle the differences Forge 1.13+ has been changed to, making Fabric the only current way to have mods added for 1.14 with the launcher and possibly in the future (including snapshots that can be modded). There is currently 1.15 snapshot support for Fabric, that being 19w35a (Better citation inclusion needed https://multimc.org/posts/forge-114.html).

The Minecraft Java Edition Launcher was supported not long after 18w50a via the Fabric Installer.

Installation instructions for the Minecraft Java Edition launcher
Fabric Installer is purported to be the tool to make instances of 1.14 Minecraft Java Edition snapshots from Minecraft versions 18w49a and onwards to be modded (public knowledge, but not accurate). Fabric API is a core mod of sorts for mods. It is best to not get these mixed up.

Fabric Installation instructions can be found on the Fabric wiki [here http://fabric.asie.pl/wiki/install].

The Fabric Installer can be installed from [modmuss50's jenkins https://jenkins.modmuss50.me/job/FabricMC/job/fabric-installer/job/master/].

A step by step process is available below:

Fabric is installed into the Minecraft Java Edition launcher from a tool on the Fabric Wiki [here http://fabric.asie.pl/use/]. This source has two MAIN methods: one for MultiMC and the vanilla Minecraft Java Edition launcher. Since this guide assumes the user will be using the vanilla launcher, select that tab and Click the 'download.jar' button for the Installer. Once it is downloaded, double-click on the file.

There are a few options to be aware of.

Once Fabric has installed its profile (it is recommended to do this the first time Fabric is installed), it will be either installed already or the user will have to select the Fabric version from the Launch Options tab. Fabric snapshots can be switched at any time from the Launch Options to update as well and use the same profile; untick the Create profile check box to do so and it will not create a profile but will install the updated Fabric snapshot desired.

The Fabric API is also needed to make mods possible to run in this instance (not all but many do require it). Fabric API is like a library mod that is required by mods that are in the Fabric environment to work. The common source to get Fabric API builds is from Curseforge.

Once Fabric API Minecraft Java Edition is downloaded and placed into the mods folder along with some mods the game will become ready for modded play. Sometimes, when Minecraft snapshot versions bump, Fabric API may need an update as well.

'''Note: Like Rift, or Forge for Minecraft 1.13.2, Fabric does not read mods in individual folders. While Forge might have been able to read folders from 1.12.2 when they were put into it, Fabric cannot.'''

Curseforge source for Fabric API builds can be found [here https://minecraft.curseforge.com/projects/fabric].

Vanilla instructions
Once a modded Fabric setup has been made, download [Optifine https://www.optifine.net/downloads], go to the downloads, and select from the preview versions and put the Optifine preview file into the mods folder alongside OptiFabric which you can be downloaded from [here https://minecraft.curseforge.com/projects/optifabric] to use for compatibility between the two. One thing to consider with the OptiFabric installation is that the preview file can be used as is (out of the box) or extracted to get the same result; this is not the case with other modloader installations as far as it is known when it comes to OptiForge or OptiRift, but without OptiFabric it will not work with a Fabric created instance; this applies to both the Preview file and the extracted method. This is unrelated to the Optifine install for vanilla 1.14.2 and only applies to Fabric modded instances for 1.14 versions. Once it is loaded up Fabric mods will work alongside Optifine.