code: function getEnterTargetState (el, stylesheet, startClass, endClass, activeClass, vm) {
    const targetState = {}
    const startState = stylesheet[startClass]
    const endState = stylesheet[endClass]
    const activeState = stylesheet[activeClass]
    // 1. fallback to element's default styling
    if (startState) {
      for (const key in startState) {
        targetState[key] = el.style[key]
        if (
          process.env.NODE_ENV !== 'production' &&
          targetState[key] == null &&
          (!activeState || activeState[key] == null) &&
          (!endState || endState[key] == null)
        ) {
          warn(
            `transition property "${key}" is declared in enter starting class (.${startClass}), ` +
            `but not declared anywhere in enter ending class (.${endClass}), ` +
            `enter active cass (.${activeClass}) or the element's default styling. ` +
            `Note in Weex, CSS properties need explicit values to be transitionable.`
          )
        }
      }
    }
    // 2. if state is mixed in active state, extract them while excluding
    //    transition properties
    if (activeState) {
      for (const key in activeState) {
        if (key.indexOf('transition') !== 0) {
          targetState[key] = activeState[key]
        }
      }
    }
    // 3. explicit endState has highest priority
    if (endState) {
      extend(targetState, endState)
    }
    return targetState
  }
docs: determine the target animation style for an entering transition.