You were asked to build a website? Does it seem it will be a larger one? It might be the case it will actually not be a website but a webapp. It might need a team to actually do it. Well, better prepare answers for these couple of questions :-)
Vision and requirements
- Is there a short document explaining why this piece of software should exist and a concise description how it would accomplish that?
- Do we have a place for collecting and prioritizing requirements?
- Are non-functional requirements written somewhere?
- Do we have a schedule or milestones which describes at least roughly what will be done when?
- Can you think of any expectations that are not described by current requirements?
- Will the customer collaborate with us on a regular basis? 
- Does anyone from the team have a specific know-how which is necessary / effective to leverage for successful completion of the project?
- Will there be a reasonable return of investment from learning and implementing new technologies? Would it be easier to hire new people with such skills?
- Are there existing systems like CMSes or APIs that needs to be integrated?
- Are there existing URL addresses or domains that needs to be redirected to new ones once the new website will be released?
- Is there a possibility that currently running systems will be negatively impacted after deploying this new application on target domain?
- How will we host our website or application? Is it possible to change it now or in the future?
- What are the pros and cons of chosen hosting environment over some more traditional choice?
- Do we have some existing analytics data?
- How many users do we expect first day, first month, first year?
- Which browsers and their versions are going to be supported?
- Would the project suffer from missing accessibility features? Is it a subject of regulation?
- Is it any good to provide users with a chatbot and is it worth the effort and possible performance issues?
- Do our users need more language mutations and other internationalization adaptations? Will we need to support a RTL language?
- Will we need to build multiple variants of the software for different users/customers?
- What is/will not be in a version control system? (like secrets, documents, etc.)
- Is there a
README.mdfile which has enough information for any developer to run the application, test it or prepare it for production?
- Does every developer have an automatic way of formatting new code? (in editor on save, precommit hook)
- Is there a process in CI that checks common code style issues for every change? (i.e. linting)
- Is there a process in place that ensures that all code is checked for quality? (code reviews, automated tests and checks)
- Does every developer know when and how a code review is needed and when it is appropriate to merge? 
- Are there some automated checks for security? 
- Is there a CI/CD pipeline that deploys every change? (continuous deployment)
- Is there a preview environment for every change and is it practical to have them?
- Do we have know-how when and how to use unit testing, integration testing or end-to-end testing?
- Do we use the right programming language for the job?
- Which framework do we use, what it solves and what problems it does not solve?
- Is there a way to generate some boring boilerplate code?
- Do we have a set of testing data?
- Can we develop against this data set without even running the database?
- Will there be only one version of the software at a time? Or will we have to support multiple versions with following maintenance?
- Does the web app handle redeployments of static assets seamlessly?
- How will the web app handle a breaking change in backend APIs while an old version of frontend code is still running on the client side?
- How is the cache on the CDN level invalidated?
- Do we send correct security headers?
- Do we send correct cache headers for HTML, data requests and assets?
- Does every page have its canonical URL? 
- Does our canonical URLs end with or without a slash? Do all redirects, sitemap items and links target this canonical URL?
- Do our domain default to www prefix?
- Do we need more control over Google results, and will we use Google search console? Will we use structured data?
- Which website analytics solution will be used? Will we try to project for missing data (blocked by users or usage before consent) and how?
- Do we have a complete sitemap?
- Do we have a robots.txt? Does it contain only necessary records?
- Do we have a page on the site for every frequent search term?
- How are we managing third party scripts? Do we need Google Tag Manager? How much will be the performance influenced by third party scripts?
- Does the application respond with correct response codes? An unknown URL should be 404 etc.
Performance, UX, a11y
- Is it OK to display loading spinners for 10 seconds on a 3G network before displaying the content?
- Will we have to do image preprocessing? Will it be a hosted solution? Do we care about resolutions and formats of those shipped images?
- Is it useful to measure Core web vitals regularly/automatically?
- Will we use progressive enhancement techniques? Does our team have knowledge to do so?
- Are we going to optimize the amount of generated CSS? How?
- Can be the desired set of fonts optimized in order to prevent Flash of unstyled fonts for most users?
- Which level of accessibility will be our target? Do we have someone who understands accessibility on this level?
- Do we have a plan for hallway usability testing? 
- How does our design process look like? How are responsibilities distributed among designers, developers, marketers, product or UX people?
- Do we need our own design system?
- Which tools and techniques we will use (preprocessing, postprocessing, feature detection, polyfills, CSS structure and layering, scoping, reset, RTL support…)?
- How will we approach responsive design? Mobile or desktop first, strict set of media queries or more fluid techniques, leverage container queries?
- Which set of icons we will use? Will it play well with our framework of choice?
- Do we have a live web with our style guide?
- Is it necessary to have a cookie consent on the website?
- In case we need a cookie consent would it be beneficial to use a third party solution? Will we use the analytics from that solution for something?
- Do we need to implement something extra in order to comply with privacy laws like GDPR?
- Where will we track tasks and bugs?
- How can our customers report bugs?
- Do we have an automated system for reporting runtime errors?
- How are our production systems backuped? When was the last time we have done a restoration from those backups?
- How often there is a change in some data or source code? For how long will our system handle the increasing frequency of change?
- Which types of changes would our architecture allow easily and which would be hard?
- Do we have a place for writing down maintenance procedures? (E.g. updating dependencies, data schema migration, adding redirects, etc.)
- Who manages credentials and how? Do we need to use an external service for them? Do we need to rotate them?
- Is there a single place (like a URL) which anybody from the team can visit and see important links or other useful information about the project? (I call this a project signpost)
- Do we need to track special events in the system, like conversions?
- Do we have a way to automatically check for links that accidentally targets PAGE NOT FOUND?
- Does our monitoring, logging and tracking solution solve real problems?
Marketing and sales
- How does marketing or sales strategy correspond to a release date of our product?
- Do we focus enough on feedback from users, developers, investors?
The bottom line
This is not an exhaustive list, although I was trying to be complete—based on my experience. BUT keep in mind that it is also critical to maximize the amount of work that does not need to be done! 
From Agile manifesto: Customer collaboration over contract negotiation ↩︎
One nice strategy for this is called Ship / Show / Ask ↩︎
For example GitHub has tools for automated checks of dependencies and static code analysis ↩︎
There are quite a few rules for canonical URLs so it is best to consult Google search documentation ↩︎
Hallway usability testing is mentioned for example in the famous Joel’s test ↩︎
Again a quote from Agile manifesto ↩︎