In odoo-9 use Style and Css to make for dynamic website. Styles are defined using standard CSS files. You can define a style as default or optional. The default styles are always active in your theme, the optional styles can be enabled or disabled by the user.
For example you can use below code in your src file.

<Customize header>
<template id="custom&#95;header" inherit&#95;id="website.layout" name="Custom Header">

  <!-- Assign an id  -->
  <xpath expr="//div[@id='wrapwrap']/header" position="attributes">
    <attribute name="id">my&#95;header</attribute>
  </xpath>

  <!-- Add an element on the top menu  -->
  <xpath expr="//div[@id='wrapwrap']/header/div" position="after">
    <div class="container">
      <div class="alert alert-info mt16" role="alert">
        <strong>Welcome</strong> in our website!
      </div>
    </div>
  </xpath>
</template>