From the Super Mario Wiki
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Semi-Solid Platforms are platforms found throughout the Super Mario series that the player can jump through from the bottom. They have a different design in each installment, first appearing in Super Mario Bros. 2. Another kind of Semi-Solid Platform is the Mushroom Platform.
Super Mario series
Super Mario Bros. 2
In Super Mario Bros. 2, Semi-Solid Platforms first appeared in World 1-1. There are four types that make an appearance: grassy (in Worlds 1, 3, and 5), sandy (in Worlds 2 and 6), snowy (in World 4), and tree-styled (in World 7).
Super Mario Bros. 3
White platforms have the special property of allowing Mario to fall into the background, behind most objects, by crouching on them for an extended time. If, as is possible in World 1-3, the goal area is reached in this state, Mario is transported to a special Toad House to receive a Magic Whistle.
Super Mario World
New Super Mario Bros. series
Semi-Solid Platforms appeared in New Super Mario Bros., New Super Mario Bros. Wii, New Super Mario Bros. 2, and New Super Mario Bros. U. While they have new, varying designs throughout the series, the most common design in all games is that consisted of small blocks placed next to each other. Certain Semi-Solid Platforms in New Super Mario Bros. 2 are based off their design from Super Mario Bros. 3. Others resemble the ones in the Yoshi's Island series, but alternate between large and small segments.
Super Mario Galaxy
Yoshi's Island series
Semi-Solid Platforms appear many times in the Yoshi's Island series, starting with Super Mario World 2: Yoshi's Island. Here, they typically resemble rows of irregular round shapes, and can also form slopes.
Super Smash Bros. series
Platforms very similar in function to Semi-Solid Platforms known as pass-through platforms appear in many stages throughout the Super Smash Bros. series. In addition to being able to be jumped through, the player can fall through pass-through platforms by quickly tilting down on the left analog stick or tapping , depending on the control method.
Semi-Solid Platforms from the Mario franchise, which function exactly like pass-through platforms, also make an appearance in the Golden Plains stage in Super Smash Bros. for Nintendo 3DS, where they use the design of the solid, tree-like platforms from New Super Mario Bros. 2. They also appear in the stage Mushroom Kingdom U where they, like the stage itself, use their design from New Super Mario Bros. U.
Super Mario Maker / Super Mario Maker for Nintendo 3DS
Semi-Solid Platforms appear as tools in Super Mario Maker and Super Mario Maker for Nintendo 3DS. In Create mode, they can be shaken into three different designs. In the Super Mario Bros. style, Semi-Solid Platforms take their default design from the otherwise solid, tree-like platforms from that game.
If the player were to crouch on the white platforms for long enough, Mario will hop slightly, referencing the special property of the white platform from Super Mario Bros. 3, though not letting Mario drop behind the platform.