While I was browsing for different anime in different genre, I came across with Yuri genre and then found a list where anime that contain yuri genre. I was shocked to see Cardcaptor Sakura on the list as anime with yuri as an additional element. I have watched Cardcaptor Sakura a lot of times but I haven't read it's manga. The only thing that comes on mind that could be the yuri element was Sakura and Tomoyo's relationship as best friends (and their mothers'). If I'm not mistaken, since I've watched Cardcaptor Sakura a lot of times when I was still a kid, there were no solid implications that Tomoyo had romantic feelings towards Sakura, meaning you can only sense friendship in their relationship (or it was something I overlooked or I didn't give deep meaning to it). Though, I'm not really sure in the manga since I've never read it. So, is there any truth to this and if it has (in anime or manga), please show reference to justify it (in any episode or chapter).
On the TV Tropes page, Tomoyo is listed as a Token Yuri Girl:
Tomoyo Daidouji in Card Captor Sakura, though it was toned down a bit in the anime. The manga actually had her telling Sakura that she loves her in that way, though it went right over the heroine's head.
So it seems that it's toned down in the anime (I didn't seem like anything other than best friends and maybe borderline infatuation in the anime), but blatant in the manga. Also noted in the CSS wiki:
While Tomoyo is in love with Sakura, she notes that she is happy to be able to be with Sakura and does not require Sakura to love her back.
According to this blog post (contains lots of other instances), it happened in Volume 2, Chapter 2:
I can't remember what episode of the anime it was but it was toward the end of the Clow Card saga. I remember it was when Sakura and Li was over at Tomoyo's house. Sakura was in the bathroom changing into one of Tomoyo's latest costumes. While Li was shyly looking towards the area where Sakura was, Tomoyo said something to li. Then, while looking into her cup she said something on the lines of, "It's better to keep some things in the heart."
At the teddy bear shop with Eriol and Li, Tomoyo told Sakura that all she wants is for the person she loves to be happy, so the person she loves doesn't have to love her back. Sakura told her that she was sure Tomoyo's beloved was very happy.
I also would say yes, but I am digging this out of the past to add some insight that it is portrayed (especially in the manga) as being a far more complex emotion than a typical "in love with" scenario.
One of the predominant themes of Clamp's works is that love has many different forms and is not limited to typical boundaries. Reflect on how many other extra-ordinary relationships and love-type affections exist in the story: Rika x her sensei (age), Touya and Yuki (gender), Mizuki x Eriol (age), Sonomi x Nadeshiko (gender, family), Fujitaka x Nadeshiko (age), Mei-lin x Syaoran (family)...
If you read other Clamp works, you will see this is one of their most common themes:
Suki (large age gap), Chobits (AI-human), X (fate), Wish (angel-human), and others - unrequited love, secret or forbidden love, love between different humanoid species...
It is apparent that all have some rather extraordinary love stories, and explore some pretty deep topics on the nature of love and the spectrum of its manifestations. Furthermore, if you look into historic Japanese literature, the theme of different kinds of love has deep roots in the cultural literature - consider Samurai/battlefield love, for example.
I would not categorize Tomoyo as a typical yuri girl, however, as was said above. Are her feelings romantic for Sakura? Certainly. Do her love feelings also incorporate strong elements of hero admiration, deep friendship, and a family bond? Absolutely. Are we given the impression that there is an element of sexual attraction? Not really. We don't see her blushing when Sakura is changing in front of her, long lingering touches, or heart pounding with physical proximity - like we do see with several other love attractions within the same series.
Granted, at their age, sexuality might be barely nascent. But typical "yuri girls" are almost invariably associated with sexual attraction regardless of age, frequently exhibiting signs of physical attraction, or even overt sexual overtures of one type or another. Is it within the yuri spectrum? I would say yes, but it's not what I would consider typical.
Also note that Tomoyo's mother Sonomi had a very similar adoration for her own cousin Nadeshiko (although hers was to the point of extreme jealousy!) and then quite obviously eventually did have a sexual relationship with Tomoyo's father (whom we never see). It is not all that uncommon for an early "crush" of a later-heterosexual female to be another female, while they are figuring out how to process these budding complex emotions.