Question: Is there a keyword in Robot Framework to ensure element has a certain class? Something like

`Element should has class element className`

Alternatively, I could check if element has a certain attribute with certain value. Former would be more suitable though, as element may contain multiple classes.

Answer: 
You could create a new keyword via XPath selectors:

```
Element should have class
    [Arguments]  ${element}  ${className}
    Wait until page contains element  ${element}[contains(@class, '${className}')]

```

Or via CSS selectors:

```
Element should have class
    [Arguments]  ${element}  ${className}
    Wait until page contains element  ${element}.${className}

```

`Wait until page contains element` could be replaced by any keyword of your liking to check if the element exists and is visible, such as `Element should be visible`.
Here's an alternative solution (though the accepted answer's CSS one is quite good), working for any kind of selector strategy:

```
Element should have class
    [Arguments]  ${locator}  ${target value}

    ${class}=       Get Element Attribute  ${locator}@class
    Should Contain  ${class}        ${target value}

```

It can be modified for any other attribute - just substitute the `@class` in `Get Element Attribute` with it (or even, make it an optional argument).