Early Deku Leaf is a sequence break that allows you to obtain the Deku Leaf before completing Dragon Roost Cavern. This sequence break allows you to easily skip the bottle and makes Dragon Roost Cavern slightly faster.
First of all, activate the Tingle Tuner. After obtaining the Wind Waker, you can do a double storage superswim to Forest Haven in order to skip having to first beat Dragon Roost Cavern. In Forest Haven, you need to activate the Deku Tree cutscene in order for the Deku Leaf to appear. After you've watched this cutscene, damage yourself down to a quarter of a heart with either Chus or Bombs. Exit the inside of Forest Haven at the lowest level. When you are outside, kill the Octorok. Walk forward until you hit the little ledge in front of you. Place a bomb and then turn around and perform a targeted slash to kill yourself and start a Zombie Hover. On the Tingle Tuner, select the Red Ting. Once you've hovered near the highest platform, you can use your Red Ting for 20 rupees to heal yourself. The unique aspect of this hover is that you do not need to actually land on the platform that you're hovering to. If you're relatively close to it when you heal yourself, you'll fall into the water, void out, and spawn back on the platform. From here, you can enter Forest Haven. When you enter, do a sidehop and C-up, line up the B Button with the spot seen in the video below, and then hold forward. If done correctly, Link should have the perfect angle for Bulb Skip, and now you can obtain the Deku Leaf. Skipping the final bulb saves 5 seconds.
It is faster to perform this hover before spawning the Deku Leaf if you perform the Deku Tree Cutscene Skip. If you fail the cutscene skip you have to redo the hover however.
As an alternative to using the first-person camera (c-up) to get an angle for jumping to the bulb, you can sidehop right then do two dry rolls, and hold up-left during the second dry roll until you jump. If done right you should land in the bulb. The downside to this is that you need to get the angle for skipping the final bulb in the short time you have before you get spit out.
If you accidentally fail the jump to the bulb, there is a way to get back up without redoing the hover. Generally when you void out somewhere you will respawn at the entrance you used. It is thus possible to use a roll clip to jump out of bounds and respawn back at the top.
By hovering with a very specific angle inside Forest Haven, you can hover up to the deku leaf and grab it while dead to obtain it early.
You can use bombs to start the hover, but it is also possible without bombs by using fall damage as seen below:
It is possible to obtain the Deku Leaf without the grappling hook and without zombie hovering. To do it, you first need to get chest storage. A good place to get chest storage is on one of the rafts in either Boating Course or Five Star Isles. With chest storage, superswim back to Forest Haven. Thanks to the alterred collision, it is relatively easy to clip under the island. Then swim under the island until you get brought back up near the entrance loadzone. Make sure not to trigger the loading zone or else you will lose chest storage and a lot of time. Thanks to chest storage once again, you can climb around the entrance and perform repeated jumpslashes up Forest Haven. You will get enough height in order to hit the respawn trigger. Then simply wait until Link drops into the water to respawn at the top of FH, skipping the grappling hook requirement for the Deku Leaf.
The strat above can also be done swordless by acquiring a deku stick. The deku stick can be used to jumpslash your way up the island with chest storage. In order to obtain a deku stick, you must lure an octorok into shooting a deku baba, and time a tingle bomb to deal the final blow. Then you can throw the stick next to the Forest Haven entrance and proceed like before.
By pulling out the Sail (or a similar cutscene item) on the frame you land on a slippery slope, it is possible to get a backflip/sidehop after the cutscene ends and before slipping off. Doing this repeatedly makes it possible to climb Forest Haven with chest storage. This method requires many frame perfect inputs and is really slow, and should never be done over the other methods.
Getting a stick is possible without the Tingle Tuner by manipulating two different octoroks into both shooting the boko baba in a very quick succession, but that is very hard/inconsistent.
Just like with chest storage, it is possible to climb Forest Haven with Door Cancel. The angle to jumpslash up the island is much more precise than with chest storage.
It is possible to heal from a zombie hover without the Tingle Tuner using a trick known as Fake Death. To do so, first get storage and store KoRL's text. Then die and zombie hover to the second to highest loading zone of Forest Haven. Once you are right above ground, pull a bomb, land and die. The death cutscene will start and KoRL's textbox will pop up on screen. Around the end of the death cutscene, close the textbox to get pushed off by the previously frozen-in-place bomb. Zombie Hover and wait for the "save the game" prompt to appear. Select don't save and then continue to get healed. You can then proceed to obtain the leaf early.
This last method of getting the deku leaf early is considered to be a TAS strat. First, get chest storage and superswim over to Bird's Peak Rock. It is possible to swim up land with chest storage and ride up the island. While riding up the island, activate a Tingle Balloon and then shoot off. You will travel backwards very high above the ocean at very high speed. Enter the loading zone at Forest Haven to obtain the leaf early.