According to Anime Filler List, the original “Cardcaptor Sakura” series is 35% filler. If you want to avoid any non-manga content, the site recommends viewers skip episodes 12 through 15, 19 through 24, 27 through 34, 36 through 38, Episode 40, Episode 43, Episode 53, 55 through 58, and 60 through 64. Most of these episodes show Sakura and her frenemy-crush Syaoran capturing various cards that weren’t in the manga. For example, in Episode 12, the first filler episode titled “Sakura’s Endless Day” (via IMDb), Sakura gets stuck in a stable time loop. The “Groundhog Day”-style loop turns out to be the work of the Time Card, which Syaoran captures for himself.
In many ways, the filler episodes of “Cardcaptor Sakura” are indistinguishable from the manga-compliant ones. In a “monster of the week” show, does it really matter if those monsters appeared in a manga first? However, if you are looking for a tighter story in line with CLAMP’s original outline of the series, then skip these filler episodes. Notably, none of the 2018 revival series is filler, according to Anime Filler List, indicating that it’s a true love letter to the fans of the manga.