I read this from fluffyponyza regarding safety of his site MyMonero.com:
There's simply no way I can control and guarantee the JavaScript delivery from the server to your computer, so you can be served poisoned JavaScript. Also, any local malware can and will steal your MyMonero private login key (we've seen evidence of this running in the wild).
Is it possible to avoid this risks of Javascript poisoning ?
Yes. In the mean time, a Chrome application have been released to largely reduce the security limits of MyMonero (that come from the concept of a web wallet rather than errors from the developer). https://chrome.google.com/webstore/detail/safer-mymonero/hcgliilmeggjhmpkfglnekbegacockei
It protects you from "JS modifications, HTML injections, HTTP downgrading and Google Tracking".
Keep in mind that it is just "Safer MyMonero" and that there still are some risks, in particular local malware as mentioned by fluffyponyza.