Google tries to render as many pages as possible. For many pages, this is completed within a few minutes. However, rendering can take up to several days. Similar to the crawl budget, a decision is made when rendering whether a page is important enough or whether sufficient resources are available. In the case of pages that are less relevant to Google, the page may not be rendered at all.

Therefore it is not only important to crawl all pages with Javascript, but also to recognise differences between the HTML version and the rendered HTML.

If a page in the HTML source text, for example, has set a noindex tag, it will not be rendered at all and a robots tag changed with JS is not taken into account.

One must take this complexity between the two HTML variants (rendered and un-rendered) into account in the indexing strategy.

The differences between the rendered HTML and the original source text are recorded with each crawl to identify necessary changes. In this way, it is possible to analyse whether there is a need for action. We show differences among others in the following elements:

• Title tags
• Canonicals
• Robots tags
• Content length
• Internal links