Imagine this: the browser and the website are like an ice skating couple. When they follow the choreography and are used to working together, the effect can be dazzling and we feel like we can watch or surf forever.
However, like any dancing couple knows: if you have performed together for a long time, you have grown accustomed to each other's peculiarities: weight, height, rhythms, accents, quirks. No new partner will ever be exactly the same, though they may be even better dancers on a simple technical level.
If you start dancing with a new partner, both may know the choreography perfectly but you'll never be prepared for how your partner was used to interact with the previous dancer. You will only discover through actually dancing together. An unexpected transfer of weight, and your skating partner hits the ice… That's why there are rehearsals.
A website needs to "partner" lots of different browsers. And browsers mostly discover through real-life site usage what odd quirks and habits the site has added to work around issues with the browsers is is used to working with. Like in dance, the oddities and habits are often the hardest things for a newcomer to adapt to.
Opera-users just hit a serious problem with blogger.com: the text of your post would disappear when submitted! It turned out to be one of these issues, an unexpected workaround against a problem with another browser hit a very obscure bug in Opera.
Perhaps one day a new browser will come along and that particular code will be a stumbling block – a peculiar accent in the dance that made the choreography flow with that specific partner… That is the rhythm of site and browser development.