When building an Email Design System you'll want to make sure that the teams creating emails are using each field in the right way. For example, ensuring they don’t use too much text for a headline or that they don’t leave a particular field blank. You can use Expectations to add this level of control to your Design System.

This means that you can tell Taxi exactly what you want from the different parts of your Email Design System, so you can do more to stop emails being exported with content missing or with content you don’t want.

For example below, there is a placeholder image that has to be changed before the email is exported from Taxi. There is a message in the editor telling you it needs to be changed and there will also be a warning if you try to export this email without changing the image. Therefore reducing the chance of this email getting sent out with a placeholder image.

To find out more information about how this will appear in your Taxi account, have a look a this help article.

To add this to your code, you use the expect attribute on fields or editables. You can use more than one Expectation per field or editable. To do this, separate them with semicolons, similar to CSS.

<editable rich name="text" default="lorem ipsum" expect="must-change;no-blank;copy-limit:30">

The values you can use in this attribute are:


If this is applied then the field has to be changed before the email is exported. For example, if you have ‘Lorem ipsum’ placeholder copy, you can use the must-change Expectation to make sure this isn’t in the email when it is exported. 

The field must be changed before the email can be exported. This Expectation can be used on all fields.

<editable name="text" default="lorem ipsum" type="text" expect="must-change"></editable>


This means that the field has to have a value input before the email is exported, it can’t be left empty. This Expectation can be used on all fields.

<editable rich="" name="text" default="lorem ipsum" expect="no-blank"></editable>


This is similar to the must-change attribute but it means that the fields in the non master version of a mailing must not be inheriting their content from the master version. This can be used on all fields.

<editable rich name="text" default="lorem ipsum" expect="must-translate"></editable>


You are able to set a limit on the number of characters that can be used in rich text fields. The field will show a warning when the copy limit is exceeded and you won’t be able to export the email.

Unlike the above Expectations, which are just a single word such as no-blank or must-change, with this Expectation you can define an exact copy-limit for any rich text field.

<editable rich name="text" default="lorem ipsum" expect="copy-limit:20"></editable>


This Expectation can be used in alignment fields. It enables you to define which alignment options you want to be available in the editor. For example, if you only wanted the choice of left and centre alignment but not right.

The allowed values are:

  • left
  • centre/center
  • right
<field name="alignment" type="align" expect="values:left centre"></field>

Existing attributes

There are several attributes that can be used in a field tag that control what options are available in that field, such as allow-custom on a colour field or allow-styles on a rich text field. These existing attributes can also be used as Expectations.


This attribute is used on dropdown colour fields when you have a set list of colours to choose from but you also want to input a custom hex code if needed. For example, allow-custom="true". You can include this in the expect attribute and the allowed values are:

  • true
  • false

If you include just allow-custom on it’s own, this is the equivalent of allow-custom:true.

<field type="colour" name="colour" expect="allow-custom:true"></field>
<field type="colour" name="colour" expect="allow-custom"></field>


Allow-none can be used on image fields and will give you the option to not include an image when there is an image field in your Design System. This can be used as a value in the expect attribute instead of as an attribute itself. You can also use allow-none:true to have the option to not include an image.

<field type="src" name="image" expect="allow-none"></field>


This is used to define what styles you want available on individual rich text fields. For example, allow-styles="bold italic". It can be used in the same way in the expect attribute with the styles separated by a space.

The allowed values are:

<field type="rich" name="text" expect="allow-styles:bold italic"></field>


This can be used on colour fields to define what formats can be used, such as hex, rgb or rgba. The default is hex if you don’t define a format and you can use one format in any colour field.

The allowed values are:

  • hex
  • rgb
  • rgba
<field type="colour" name="colour" expect="format:rgb"> </field>

When the expect attribute includes something which is also set with a custom attribute, then you will see a warning when you upload your HTML. For example, if allow-custom is set as an attribute but also as part of the expect attribute. 

If the expect attribute contradicts another attribute in that field then there will be an error message when you upload your HTML. For example, allow-custom="true" and expect="allow-custom:false" in the same field would cause an error message.

Setting Expectations for images 

Images are one of the most powerful design elements in an email marketers’ toolkit — but even the most beautiful image can’t shine if it’s uploaded in the wrong format, causes slow load times, or gets distorted. Translate your guidelines and email marketing best practices around imagery, into Taxi by adding the following set of Expectations to your image src fields and stop any mistakes before they happen.


Use the max-file-size Expectation to ensure that images never exceed a certain file size. Whether you’re using static images or animated GIFs, setting a limit for your files sizes helps make sure that your imagery isn’t causing slow load times. File sizes must be set in kb, so if you’d like to set a file size limit of 1MB for your images, here’s how that would look:

<field type="src" name="image" expect="max-file-size:1000"></field>


Specify what image file formats are supported within your Email Design System with this Expectation.  If you’d like to allow multiple image formats, let’s say gif, jpeg, and png file types, you can separate them with a space like this:

<field type="src" name="image" expect="format:gif jpeg png"></field>


With the aspect Expectation you can specify what aspect ratio requirements your images must meet. For example, this is how you’d make sure that all uploaded images have a 16:9 aspect ratio.

<field type="src" name="image" expect="aspect:16by9"></field>


Use the width Expectation to make sure all images are exactly as wide as you’ve specified. The width is set in pixels.

<field type="src" name="image" expect="width:600"></field>


Use the height Expectation to make sure all images you add to your emails are exactly as high as you’ve specified. The height is set in pixels.

<field type="src" name="image" expect="height:400"></field>


Use the min-width Expectation to make sure all images are at least as wide as you’ve specified. The min-width is set in pixels.

<field type="src" name="image" expect="min-width:600"></field>

Max-width Use the max-width Expectation to make sure all images don’t exceed the width you’ve specified. The maximum width is set in pixels.

<field type="src" name="image" expect="max-width:700"></field>

If you’d like to provide a range for an allowed image width, you can combine the two attributes. For example, here’s how you’d set an Expectation that requires all image to be between 600 and 700px wide:

<field type="src" name="image" expect="min-width:600; max-width:700"></field>


Use the min-height Expectation to make sure all images are at least as high as you’ve specified. The min-height is set in pixels.

<field type="src" name="image" expect="min-height:600"></field>


Use the max-height Expectation to make sure all images don’t exceed the height you’ve specified. The maximum height is set in pixels.

<field type="src" name="image" expect="max-height:700"></field>

You can use the same approach as above, and use min-height and max-height together to create a range of heights that the image can be.

All of the width and height Expectations are related to the size of the image file or url you add to your email, not the number you add to the width or height fields for an image.

Other considerations

These Expectations can be used on editables or fields. If used on an editable, all fields within that editable will use that Expectation. If used on a field, the Expectation will only apply to this field, and if that field is within an editable that has a different Expectation, the field Expectation will take precedence.

Not all Expectations can apply to all fields, for example you can’t have a copy limit on a colour field. If a field is inside an editable that uses an Expectation that can’t be used, the Expectation will be ignored for that field. If no fields in the editable can use the Expectation then you will see a warning message when you upload your HTML.